Add SKImageFilter.CreateCrop() - Fixes #3938

[mattleibow]

Implements SKImageFilter.CreateCrop(SKRect, SKShaderTileMode, SKImageFilter?) to wrap the modern Skia SkImageFilters::Crop API.

This is the modern replacement for the deprecated cropRect parameter pattern on individual filters. From Skia's header comment:

The optional CropRect argument for many of the factories is equivalent to creating the filter without a CropRect and then wrapping it in ::Crop(rect, kDecal).

Closes #3938

Dependencies

• Depends on Add sk_imagefilter_new_crop C API skia#261 (C API changes)

Changes

C API (externals/skia submodule - mono/skia#261)

• Added sk_imagefilter_new_crop to sk_imagefilter.h
• Implemented wrapper in sk_imagefilter.cpp

C# Wrapper

• Added CreateCrop overloads to SKImageFilter.cs:
  • CreateCrop(SKRect) - defaults to Decal mode, null input
  • CreateCrop(SKRect, SKShaderTileMode) - null input
  • CreateCrop(SKRect, SKShaderTileMode, SKImageFilter?) - full control
• Regenerated SkiaApi.generated.cs

Gallery Sample

Added ImageFilterCropSample.cs demonstrating:

• Interactive crop rect adjustment (Left, Top, Right, Bottom sliders)
• All tile modes selectable (Decal, Clamp, Repeat, Mirror)
• Optional blur composition to show filter chaining
• Visual crop rect outline for clarity

Tests

Added 6 comprehensive tests in SKImageFilterTest.cs that validate:

• ✅ API returns non-null for basic usage
• ✅ All tile modes work (Decal, Clamp, Repeat, Mirror)
• ✅ Null input handling (uses source image)
• ✅ Composition with other filters (blur)
• ✅ Pixel-level rendering verification: Validates that pixels outside crop rect remain untouched (white background preserved) and pixels inside crop rect are rendered correctly (red color applied), proving Decal mode transparency behavior works as expected

Testing

All 5712 tests passed:

• Passed: 5712
• Skipped: 13
• Failed: 0

Native build completed successfully on macOS ARM64.

Merge commit message

[api] Add SKImageFilter.CreateCrop() (#4229)

Context: https://github.com/mono/SkiaSharp/issues/3938
Fixes: https://github.com/mono/SkiaSharp/issues/3938
Changes: https://github.com/mono/skia/pull/261

Exposes Skia's `SkImageFilters::Crop()` API, which crops an image filter
output to a rectangle. This is the modern replacement for the deprecated
`cropRect` parameter pattern that existed on individual filter factories.

From Skia's header comment:
> The optional CropRect argument for many of the factories is equivalent
> to creating the filter without a CropRect and then wrapping it in
> ::Crop(rect, kDecal).

The new API allows explicit composition of filters with crop boundaries,
making the intent clearer and enabling more flexible filter chains. For
example, applying a blur then cropping the result is now expressed as:

    var blur = SKImageFilter.CreateBlur(5, 5);
    var cropped = SKImageFilter.CreateCrop(rect, SKShaderTileMode.Decal, blur);

instead of passing cropRect parameters deep into each filter factory.

Changes:
  * C API: added sk_imagefilter_new_crop to externals/skia C shim
  * C# wrapper: added three CreateCrop overloads to SKImageFilter
  * Gallery sample: interactive ImageFilterCropSample with all tile modes
  * Tests: six comprehensive tests including pixel-level rendering validation

All 5712 existing tests continue to pass.

Co-authored-by: Matthew Leibowitz <mattleibow@live.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

[github-actions]

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[github-actions]

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