[web] Support FragmentShader.getImageSampler on CanvasKit and Skwasm - #195854

Implements FragmentShader.getImageSampler(name) on the CanvasKit and Skwasm web renderers. Before this change, both backends threw UnsupportedError, so web apps could bind float and matrix uniforms by name but not image samplers.

The sampler index lookup now lives in ShaderData.getImageSamplerIndex, which both backends share. It is a port of the native _getImageSamplerIndex: the index counts only sampler uniforms, and the error messages are the same as native. BackendFragmentShader now exposes a plain int getImageSamplerIndex(String name) query instead of returning a slot. EngineFragmentShader builds an EngineImageSamplerSlot whose set calls EngineFragmentShader.setImageSampler, so binding through a slot gets the same sampler disposal, finalizer handling, and picture image retention as calling setImageSampler directly.

Fixes #194290

Behavioral Changes

• FragmentShader.getImageSampler(name) on CanvasKit and Skwasm:
  • Before: always threw UnsupportedError('getImageSampler is not supported on the web.').
  • After: returns an ImageSamplerSlot. Its name is the queried name, and its shaderIndex is the 0-based position of that sampler among the shader's sampler uniforms (float uniforms are not counted). slot.set(image) does the same thing as shader.setImageSampler(slot.shaderIndex, image).
• Invalid names now throw ArgumentError with the native messages:
  • a non-sampler uniform: Uniform "<name>" is not an image sampler.
  • an unknown name: No uniform named "<name>".
• Calling slot.set after the shader is disposed fails the same debug assertion as setImageSampler.
• Web Impeller (wimp) does not implement runtime effects yet ([wimp] Implement custom fragment shaders for wimp #175431), so nothing changes there.

Where to focus

• Where the slot is built: the backend only answers a name-to-index query, and the frontend EngineFragmentShader builds the slot. That keeps image lifetime bookkeeping (samplerImages, which the picture image tracker reads, disposal of the replaced sampler, and finalizers) in one place. A slot built by the backend that called the backend's setImageSampler directly would skip that bookkeeping. This is also why the sampler slot is built in a different layer from the float uniform slots.
• Index semantics: ShaderData.getImageSamplerIndex counts only SampledImage uniforms that come before the match. That matches the index space that CkFragmentShader.setImageSampler (samplers[index]) and SkwasmFragmentShader.setImageSampler (child shader index) already use.
• CkFragmentProgram now keeps its ShaderData, as SkwasmFragmentProgram already did, and exposes uniforms, floatCount and textureCount as getters.
• Tests: a new two_samplers SkSL fixture puts samplers between float uniforms. It covers index ordering and binding in reverse declaration order through real pixels.
• Files to review first:
  1. lib/src/engine/primitives/shader.dart
  2. lib/src/engine/shader_data.dart
  3. test/ui/fragment_shader_test.dart (the getImageSampler group)

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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 in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• 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.

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