Make compileShader() retry without sksl if it fails with sksl. (#183146)

Updates `compileShader()` in `shader_compiler.dart` to retry `impellerc`
without sksl if calling it with sksl fails.

Fixes #182400

<details><summary>Running an app with a non-sksl-compatible
shader:</summary>

```
❯ flutter run -d mac
Launching lib/main.dart on macOS in debug mode...
warning: Shader `/Users/bensonluk/projects/hello_flutter/shaders/simple.frag` is incompatible with SkSL. This shader will not load when running with the Skia backend.
impellerc failure: There was a compiler error: SkSL does not support array initializers: /Users/bensonluk/projects/hello_flutter/shaders/simple.frag:12
Project /Users/bensonluk/projects/hello_flutter built and packaged successfully.
Building macOS application...
✓ Built build/macos/Build/Products/Debug/hello_flutter.app
2026-03-03 11:38:35.093 hello_flutter[76976:83279294] Running with merged UI and platform thread. Experimental.
Failed to foreground app; open returned 1
Syncing files to device macOS...                                    39ms

Flutter run key commands.
r Hot reload. 🔥🔥🔥
R Hot restart.
h List all available interactive commands.
d Detach (terminate "flutter run" but leave application running).
c Clear the screen
q Quit (terminate the application on the device).

A Dart VM Service on macOS is available at: http://127.0.0.1:57483/Nr_RN4azQw8=/
The Flutter DevTools debugger and profiler on macOS is available at: http://127.0.0.1:57483/Nr_RN4azQw8=/devtools/?uri=ws://127.0.0.1:57483/Nr_RN4azQw8=/ws
```
</details>

<details><summary>Running an app with multiple non-sksl-compatible
shaders with longer error messages:</summary>

```
❯ flutter run -d mac
Launching lib/main.dart on macOS in debug mode...
warning: Shader `/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_arbitrary.frag` is incompatible with SkSL. This shader will not load when running with the Skia backend.
impellerc failure: Compilation failed for target: SkSL
"/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_arbitrary.frag":
Compiled to invalid SkSL:
        // This SkSL shader is autogenerated by spirv-cross.

        float4 flutter_FragCoord;

        uniform vec2 uSize;
        uniform vec2 uForegroundSize;
... (truncated 426 lines)
SkSL Error:
        error: 58: loop index initializer must be a constant expression
                for (int x = _703; x <= sampleRadius; x++)
                     ^^^^^^^^^^^^
        error: 55: loop index initializer must be a constant expression
            for (int y = _692; y <= sampleRadius; y++)
                 ^^^^^^^^^^^^
... (truncated 350 lines)
Full "liquid_glass_arbitrary" error output written to "/tmp/flutter_4Te755VI/impellerc_verbose_error.txt"

warning: Shader `/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_geometry_blended.frag` is incompatible with SkSL. This shader will not load when running with the Skia backend.
impellerc failure: Compilation failed for target: SkSL
"/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_geometry_blended.frag":
Compiled to invalid SkSL:
        // This SkSL shader is autogenerated by spirv-cross.

        float4 flutter_FragCoord;

        uniform vec2 uSize;
        uniform vec4 uOpticalProps;
... (truncated 229 lines)
SkSL Error:
        error: 111: initializers are not permitted on arrays (or structs containing arrays)
            float param_2[96] = shapeData;
                                ^^^^^^^^^
        error: 112: unknown identifier 'param_2'
            float result = FLT_flutter_local_getShapeSDFFromArray(param, param_1, param_2);
                                                                                  ^^^^^^^
... (truncated 185 lines)
Full "liquid_glass_geometry_blended" error output written to "/tmp/flutter_3qE1Q4aI/impellerc_verbose_error.txt"

warning: Shader `/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_filter.frag` is incompatible with SkSL. This shader will not load when running with the Skia backend.
impellerc failure: Compilation failed for target: SkSL
"/Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/lib/assets/shaders/liquid_glass_filter.frag":
Compiled to invalid SkSL:
        // This SkSL shader is autogenerated by spirv-cross.

        float4 flutter_FragCoord;

        uniform vec2 uSize;
        uniform vec4 uGlassColor;
... (truncated 426 lines)
SkSL Error:
        error: 121: initializers are not permitted on arrays (or structs containing arrays)
            float param_2[96] = shapeData;
                                ^^^^^^^^^
        error: 122: unknown identifier 'param_2'
            float result = FLT_flutter_local_getShapeSDFFromArray(param, param_1, param_2);
                                                                                  ^^^^^^^
... (truncated 488 lines)
Full "liquid_glass_filter" error output written to "/tmp/flutter_OQtgOkth/impellerc_verbose_error.txt"
Project /Users/bensonluk/projects/flutter_liquid_glass/packages/liquid_glass_renderer/example built and packaged successfully.Command PhaseScriptExecution emitted errors but did not return a nonzero exit code to indicate failure
Building macOS application...
✓ Built build/macos/Build/Products/Debug/liquid_glass_renderer_example.app
2026-03-03 11:39:31.435 liquid_glass_renderer_example[77352:83282562] Running with merged UI and platform thread. Experimental.
Failed to foreground app; open returned 1
flutter: Initializing logger: lgr
flutter: lgr.render.layer > WARNING: LiquidGlassLayer is only supported when using Impeller at the moment. Falling back to FakeGlass for LiquidGlassLayer. To prevent this warning, enable Impeller, or set LiquidGlassLayer.fake to true before you use liquid glass widgets on Skia.
flutter: lgr.render.layer > WARNING: LiquidGlassLayer is only supported when using Impeller at the moment. Falling back to FakeGlass for LiquidGlassLayer. To prevent this warning, enable Impeller, or set LiquidGlassLayer.fake to true before you use liquid glass widgets on Skia.
Syncing files to device macOS...                                    47ms

Flutter run key commands.
r Hot reload. 🔥🔥🔥
R Hot restart.
h List all available interactive commands.
d Detach (terminate "flutter run" but leave application running).
c Clear the screen
q Quit (terminate the application on the device).

A Dart VM Service on macOS is available at: http://127.0.0.1:57532/tQZkdNat5B0=/
The Flutter DevTools debugger and profiler on macOS is available at: http://127.0.0.1:57532/tQZkdNat5B0=/devtools/?uri=ws://127.0.0.1:57532/tQZkdNat5B0=/ws
```
</details>

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there for submitting PRs.
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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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