[Flutter GPU] Add r32Float and remove Apple-only XR pixel formats

[bdero]

Tidies the Flutter GPU PixelFormat surface.

Adds r32Float, a single-channel 32-bit float format that every Impeller backend (Metal, Vulkan, OpenGLES) already supports but that was never exposed to the Flutter GPU API.

Removes b10g10r10XR, b10g10r10XRSRGB, and b10g10r10a10XR. These Apple extended-range formats only function on Metal; they resolve to undefined or unsupported on the Vulkan and OpenGLES backends, so they were broken anywhere off Apple platforms (and I'm not exercising them in either Flutter GPU's internal tests nor flutter_scene's external goldens). They remain available internally within Impeller, of course. Only the public Flutter GPU surface drops them.

The portable successor for wide-gamut output is a standard rgb10a2Unorm format, which can be added down the road upon user demand. Flutter GPU is experimental, so this is a minor breaking change to its API with no known users.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request replaces several Apple-only extended-range pixel formats (kB10G10R10XR, kB10G10R10XRSRGB, and kB10G10R10A10XR) with a single-channel 32-bit floating-point format (kR32Float / r32Float) in the Flutter GPU API. It also maps the removed formats to kUnknown and adds a unit test to verify the allocation of the new r32Float texture format. One review comment suggests adding documentation to the newly introduced r32Float enum value in Dart, which aligns with the repository style guide.

[gaaclarke]

Your call but those formats are the only real viable option for wide gamut on apple platforms. Floating point formats have problems with performance and are incompatible with rgba8 fragment shaders (since they don't implicitly clamp).

[bdero]

Yeah especially since we're about to land a direct way to query for supported textures, I think it'll be good to start supporting more formats that are platform specific/specialized over time as users come to demand them... would like to avoid being a "lowest common denominator" API over the longterm.

I just don't have any tests (or external renderers) directly exercising them today. So I don't actually know if they work in Flutter GPU end-to-end right now. 🤷‍♂️

[auto-submit]

autosubmit label was removed for flutter/flutter/187069, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.