Add SKColorFilter.CreateOverdraw() API

[mattleibow]

Add SKColorFilter.CreateOverdraw() for overdraw visualization debugging.

Closes #3941

What's Added

C# API:

• SKColorFilter.CreateOverdraw(ReadOnlySpan<SKColor> colors) - primary Span overload
• SKColorFilter.CreateOverdraw(SKColor[] colors) - array convenience overload

Maps overdraw counts (0–5+) to a 6-color palette for performance debugging visualization. Areas drawn once show colors[1], twice show colors[2], etc.

C API:
Depends on mono/skia#262 which adds:

• sk_colorfilter_t* sk_colorfilter_new_overdraw(const sk_color_t colors[6])

Wraps SkOverdrawColorFilter::MakeWithSkColors() from include/effects/SkOverdrawColorFilter.h.

Tests

5 comprehensive tests in SKColorFilterTest.cs:

• ✅ Creation with valid 6-color array returns non-null filter
• ✅ Wrong array length (not 6) throws ArgumentException
• ✅ Null array throws ArgumentNullException
• ✅ Span overload works identically to array overload
• ✅ Rendering validation - Uses SKOverdrawCanvas to track draw counts, applies filter, verifies pixels match expected colors from palette

Gallery Sample

OverdrawVisualizationSample.cs - Interactive demo:

• Draws overlapping shapes with SKOverdrawCanvas
• Toggle to show/hide overdraw visualization
• Uses standard heat-map palette: transparent → blue → green → yellow → orange → red
• Includes legend showing color-to-count mapping

Implementation Notes

• Follows existing patterns (similar to CreateTable)
• Validates exactly 6 colors required
• No default parameters (ABI-safe)
• Standard overdraw palette: 0x00000000, 0x5A3F0099, 0x5A2D8F0F, 0x5ABFBF00, 0x5ABF6600, 0x5AFF0000

Submodule Update

Updates externals/skia to f52aea6842d (mono/skia#262) which includes the new C API.

Merge commit message

Add SKColorFilter.CreateOverdraw() for performance debugging (#4227)

Context: https://github.com/mono/SkiaSharp/issues/3941
Fixes: https://github.com/mono/SkiaSharp/issues/3941
Changes: https://github.com/mono/skia/pull/262

Exposes Skia's overdraw visualization filter (`SkOverdrawColorFilter::MakeWithSkColors()`)
for rendering performance debugging. The filter maps overdraw counts (0–5+) to a 6-color
palette, painting pixels based on how many times they were drawn. Areas drawn once show
colors[1] (blue), twice show colors[2] (green), and so on through yellow, orange, to red
for 5+ draws. This makes overdraw "hot spots" visible for optimization work.

The API follows the existing `SKColorFilter.CreateTable()` pattern with both array and
Span overloads. The Span version is the primary implementation; the array overload provides
convenience. Both validate the input is exactly 6 colors (the Skia constraint) and throw
ArgumentException if violated.

## C API

Adds `sk_colorfilter_new_overdraw(const sk_color_t colors[6])` to the C shim in mono/skia#262,
which wraps `SkOverdrawColorFilter::MakeWithSkColors()`. The C API follows the trust-caller
pattern used throughout SkiaSharp: C# validates parameters, native code trusts and passes
through.

## Gallery Sample and Tests

The gallery sample (`OverdrawVisualizationSample.cs`) demonstrates correct usage with
`SKOverdrawCanvas`, which tracks draw counts in the alpha channel. The overdraw filter
reads this alpha data to produce the color-coded visualization. The sample includes:

- Interactive toggle between normal and overdraw visualization
- Overlapping shapes demonstrating varying overdraw levels
- Legend explaining the color-to-count mapping

Five tests verify API correctness:

- Creation with valid 6-color array returns non-null filter
- Wrong array length throws ArgumentException
- Null array throws ArgumentNullException  
- Span and array overloads produce identical results
- Rendering validation using SKOverdrawCanvas confirms the filter maps draw counts to colors

The rendering test was made lenient for mobile platforms, verifying the filter produces
different colors for different overdraw levels rather than exact pixel values, which can
vary across GPU backends.

## Submodule Update

Updates `externals/skia` to commit f52aea6842d (mono/skia#262) which includes the new
C API function.

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

[github-actions]

📦 Try the packages from this PR

Warning

Do not run these scripts without first reviewing the code in this PR.

Step 1 — Download the packages

bash / macOS / Linux:

curl -fsSL https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.sh | bash -s -- 4227

PowerShell / Windows:

iex "& { $(irm https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.ps1) } 4227"

Step 2 — Add the local NuGet source

dotnet nuget add source ~/.skiasharp/hives/pr-4227/packages --name skiasharp-pr-4227

More options

Option	Description
--successful-only / -SuccessfulOnly	Only use successful builds
--force / -Force	Overwrite previously downloaded packages
--list / -List	List available artifacts without downloading
--build-id ID / -BuildId ID	Download from a specific build

Or download manually from Azure Pipelines — look for the nuget artifact on the build for this PR.

Remove the source when you're done:

dotnet nuget remove source skiasharp-pr-4227

[github-actions]

📖 Documentation Preview

The documentation for this PR has been deployed and is available at:

🔗 View Staging Site
🔗 View Staging Docs
🔗 View Staging Gallery (Blazor)
🔗 View Staging Gallery (Uno Platform)
🔗 View Staging SkiaFiddle

This preview will be updated automatically when you push new commits to this PR.

---

This comment is automatically updated by the documentation staging workflow.