(Un)premultiply colors in begin/end modify image + another fix

[Equbuxu]

PixiEditor PR: PixiEditor/PixiEditor#1681

[flabbet]

looks good to me, requesting clanker for obvious mistakes

[Copilot]

Pull request overview

This PR updates the shader generation pipeline to work with straight-alpha colors during shader execution (by unpremultiplying sampled surface colors) and includes a parsing fix in the Skia shader implementation to correctly detect uniform types when the uniform name overlaps with layout/type tokens (e.g., layout(color) ... color).

Changes:

• Unpremultiply sampled half4 surface colors in ShaderBuilder.Sample(...) before returning them to shader expressions.
• Add a built-in Unpremultiply shader function (and a GetUnpremultiply(...) wrapper) to support the new sampling behavior.
• Fix SkiaShaderImplementation.TryDetectType(...) by removing only the last occurrence of the uniform name when analyzing the declaration string.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/Drawie.Backend.Skia/Implementations/SkiaShaderImplementation.cs	Fixes uniform type detection for edge cases where the uniform name appears in layout/type tokens.
src/Drawie.Backend.Core/Shaders/Generation/ShaderBuilder.cs	Unpremultiplies sampled surface colors before returning them to shader expressions.
src/Drawie.Backend.Core/Shaders/Generation/BuiltInFunctions.cs	Adds an Unpremultiply built-in shader function used by ShaderBuilder.Sample(...).

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