…ata (flutter#185879)

Flutter GPU shader bundles serialize uniform struct field metadata via a
flatbuffer schema (`shader_bundle.fbs`) that carries per-field type and
size information, but does not carry the underlying vector/column
dimensions. This is enough to bind uniforms on Metal and Vulkan, but it
isn't enough to distinguish shapes that share the same
`element_size_in_bytes`, most notably `vec4` vs `mat2`. The GLES backend
uses `ShaderStructMemberMetadata::float_type` to pick the right
`glUniform*` call, and the bundle loader had no way to populate it.

PR flutter#182229 ("Turns on most of fragment_shader_test.dart for opengles")
introduced `ShaderFloatType`, the GLES validation that requires it, and
the fix for runtime effect shaders (`runtime_effect_contents.cc`), but
the equivalent fix for the Flutter GPU shader bundle loader
(`lib/gpu/shader_library.cc`) was not made. As a result, every
flutter_gpu app with a uniform block crashes the GLES backend on all
platforms (Windows ANGLE, Linux GLES, Android GLES) with:

```
[ERROR:flutter/impeller/renderer/backend/gles/buffer_bindings_gles.cc(409)]
Break on 'ImpellerValidationBreak' to inspect point of failure: Float uniform should have a float type.
```

## Approach

The size-only signal already carried by the bundle is not sufficient to
derive `float_type` correctly for `mat2` and `mat3`, which would
otherwise collide with `vec2` arrays and `vec3` arrays. (impellerc
serializes a `mat2` uniform with `element_size_in_bytes = 8` and
`array_elements = 2`, which is the same shape as a length-2 `vec2`
array. Same story for `mat3`.) To fix this without regressing
matrix-typed uniforms, this PR extends the schema to carry `vec_size`
and `columns` (matching what `ShaderInput` already does for vertex
inputs), and uses them to derive `float_type` in the bundle loader via
the same shape-based dispatch that runtime effects already use.

The shader bundle format version is bumped from 1 to 2 because old
bundles do not carry the new fields. Old bundles loaded against a new
engine fail with a clear error pointing at the impellerc/SDK version
mismatch, rather than silently rendering with the wrong `glUniform*`
call.

## Tests

- `shader_bundle_unittests.cc`: verify the bundle carries `vec_size` and
`columns` for the existing `mvp` (mat4) and `color` (vec4) test
fixtures. Exhaustively cover `DeriveShaderFloatType` for float scalars,
all vector sizes, all square matrices, non-square shapes, non-float
types, and zero (legacy bundle) values.
- `buffer_bindings_gles_unittests.cc`: regression guard asserting that
the GLES backend rejects a float uniform that arrives without
`float_type` populated. If a future change forgets to populate
`float_type` in any code path (shader bundle loader, runtime effects, or
anything else), this test catches it at unit-test time instead of at
runtime.
- `gpu_test.dart`: the eight re-enabled tests cover triangle rendering,
polygon mode, MSAA, indexed draw, stencil, cull mode, scissor, and
viewport on the opengles backend, end-to-end through the bundle loader.

## Issues

Fixes flutter#184953
Fixes flutter#183530

May also resolve the following issues, all of which surface as GLES
uniform binding failures and share the same root-cause family. To be
verified against the linked reproducers after merge:

- flutter#176875
- flutter#164438
- flutter#164745

This supersedes the smaller fix in
[`bdero/flutter-gpu-float-types`](https://github.com/bdero/flutter/tree/bdero/flutter-gpu-float-types)
(draft flutter#185874), which derived `float_type` from `element_size_in_bytes`
only and would have misclassified `mat2`/`mat3` as `vec2`/`vec3`.