[Impeller] Validate GLES texture units against the combined limit (#189332)

Fixes #189331.

The GLES backend allocates fragment stage texture units after the vertex
stage's, but validated the running unit index against the per-stage
maximum. On a driver reporting the minimum 16 fragment units (ANGLE on
D3D11), a draw using all 16 fragment samplers plus a vertex stage
texture failed validation and the render pass aborted, crashing
skinned-mesh rendering in Flutter GPU apps on Windows.

Texture units are a combined resource in GL; the per-stage limits bound
only how many samplers one stage references. This validates the unit
index against the combined limit and the per-stage sampler count against
the per-stage limit. Adds unit tests for the previously rejected case
and both overflow cases.

## Pre-launch Checklist

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- [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.