Fix cached color glyph paint replay and add a Paint overload with a text cache

[JimBobSquarePants]

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Description

Fixes the wrong color of cached color emoji glyphs when a shared DrawingTextCache replays them at a new position, and adds a Paint overload that takes a DrawingTextCache.

Bug. WebGPUWindowDemo scrolls a text block through a shared cache. The emoji in the block rendered flat brown except near one screen position. The same output appears on the CPU backend, so the cause is in the text renderer, not the GPU.

Cause. Layered color glyphs re-create their paint brushes on replay. RichTextGlyphRenderer stored the device-space glyph origin in the cache and appended the device-space delta after the drawing transform. When DrawText(TextBlock, point) passes its origin through DrawingOptions.Transform, that offset was applied twice. The radial gradient moved off the glyph and the pad color filled the face.

Fix. The cache now stores the glyph origin before the drawing transform, and the replay applies the delta before the drawing transform. The replayed brush then matches a fresh build at the new position exactly.

API. Paint(IImageProcessingContext, DrawingOptions, DrawingTextCache, CanvasAction) passes an application-owned cache to every frame canvas. PaintProcessor gets a matching constructor and a TextCache property. This mirrors the CreateCanvas overloads and lets the Image.Mutate path share a cache.

Tests.

• DrawingTextCacheTests.LayeredGlyph_ReplayAtNewPositionMatchesFreshBuild draws a color emoji through a shared cache at one point, then at a second point, and compares exactly against a fresh-cache draw at the second point. It fails on the old code and passes with the fix.
• ProcessWithCanvas.CanvasActionWithOptionsAndTextCache checks that the processor receives the cache.

Run	Result
Cache, paint and text tests, net10.0	180 pass
WebGPU and issue tests, net10.0	473 pass
All of the above, net8.0	608 pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80%. Comparing base (ef8c9c8) to head (f30b3ba).

Additional details and impacted files

@@              Coverage Diff              @@
##           release/3.1.x    #422   +/-   ##
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- Coverage             80%     80%   -1%     
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  Files                232     232           
  Lines              27392   27407   +15     
  Branches            3120    3121    +1     
=============================================
+ Hits               21969   21980   +11     
  Misses              4535    4535           
- Partials             888     892    +4     

Flag	Coverage Δ
unittests	80% <100%> (-1%)	⬇️

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