Reverts "[Flutter GPU] Add explicit draw counts (#186639)" (#187170)

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Reverts: flutter/flutter#186639
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Initiated by: cbracken
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Reason for reverting: doesn't compile on mac, or Linux.

Mac:
```
../../flutter/testing/dart/gpu_shader_reload_test.dart:64:36: Error: Too many positional arguments: 1 allowed, but 2 found.
Try removing the extra positional arguments.
  state.renderPass.bindVertexBuffer(
                                   ^
../../flutter/testing/dart/gpu_shader_reload_test.dart:72:24: Error: Too few positional arguments: 1 requ
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Original PR Author: bdero
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Reviewed By: {walley892}
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This change reverts the following previous change:
Moves the vertex and index count off the buffer-binding calls and onto the draw call.

A simple (breaking) change that fixes a very silly design flaw that I built into the API near the very beginning. Moves Flutter GPU draws towards a design that matches virtually all modern graphics APIs, including today's Impeller!
Now is the right time to make changes like this to Flutter GPU, since the API is not yet considered stable.

Before, `bindVertexBuffer` and `bindIndexBuffer` each took a count, and a single argument-less `draw()` guessed vertex versus index based on whether an index buffer happened to be bound.

Sooooo many reasons this was the wrong design:

- **Wrong object.** A draw count belongs on the draw, not on a buffer binding.
- **Implicit kind.** Indexed vs. non-indexed was inferred from what happened to be bound, not stated.
- **Colliding counts.** Vertex and index counts shared one field; the vertex count was dropped when an index buffer was bound.
- **Slot-0-only.** The count came from vertex slot 0; values on other slots were silently ignored.
- **Opaque draws.** `draw()` told you nothing; you had to trace every bind to know what it drew.
- **Sticky state.** The count leaked across draws in a pass.
- **Redundant re-binds.** Changing only the count forced re-binding the buffer.
- **No room to grow.** Nowhere natural for per-draw params like `instanceCount` or `baseVertex`.
- **Unconventional.** WebGPU, Vulkan, Metal, and D3D all put counts on the draw.
- **Late errors.** Bad counts failed at `draw()` time, far from the mistaken line.

Now:
- `bindVertexBuffer` and `bindIndexBuffer` only bind buffers.
- `draw(vertexCount)` does a non-indexed draw.
- `drawIndexed(indexCount)` does an indexed draw.

Instanced draws can be landed later via `draw` and `drawIndexed` gaining an `instanceCount` parameter later on, assuming the Impeller-side instancing support in #186653 lands.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
- [x] I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my 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.
- [ ] All existing and new tests are passing.


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[Flutter Style Guide]: https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
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[Discord]: https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]: https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

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Co-authored-by: auto-submit[bot] <flutter-engprod-team@google.com>