WIP. Expand DrawingBackend API and Canvas.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

WIP - Still lots of normalization and testing to do!

This PR expand on the backend drawing API and drawing canvas to normalize usage throughout the library and to better allow custom backends both CPU and GPU.

There's an experimental WebGPU backend in the repo but it's nowhere near production ready so serves simply as a dogfooding tool for the API design. Our new CPU backend is more than fast enough for most usage.

[codecov]

Codecov Report

❌ Patch coverage is 89.51856% with 209 lines in your changes missing coverage. Please review.
✅ Project coverage is 85%. Comparing base (e7aa1b6) to head (6fbc481).

Files with missing lines	Patch %	Lines
...p.Drawing/Processing/Backends/DefaultRasterizer.cs	91%	52 Missing and 31 partials ⚠️
...eSharp.Drawing/Processing/DrawingCanvas{TPixel}.cs	85%	58 Missing and 22 partials ⚠️
...awing/Processing/Backends/DefaultDrawingBackend.cs	81%	12 Missing and 6 partials ⚠️
...Sharp.Drawing/Processing/Backends/NativeSurface.cs	25%	9 Missing ⚠️
...awing/Processing/Backends/DrawingCoverageHandle.cs	37%	5 Missing ⚠️
...g/Processing/Backends/CanvasRegionFrame{TPixel}.cs	77%	3 Missing and 1 partial ⚠️
src/ImageSharp.Drawing/Helpers/ArrayExtensions.cs	75%	1 Missing and 1 partial ⚠️
...Sharp.Drawing/Helpers/ThreadLocalBlenderBuffers.cs	89%	1 Missing and 1 partial ⚠️
...rp.Drawing/Processing/Backends/CompositionBatch.cs	88%	2 Missing ⚠️
src/ImageSharp.Drawing/Processing/ImageBrush.cs	87%	2 Missing ⚠️
... and 2 more

Additional details and impacted files

@@         Coverage Diff          @@
##           main   #377    +/-   ##
====================================
  Coverage    84%    85%            
====================================
  Files       101     83    -18     
  Lines      4529   4950   +421     
  Branches    654    677    +23     
====================================
+ Hits       3849   4210   +361     
- Misses      540    574    +34     
- Partials    140    166    +26     

Flag	Coverage Δ
unittests	85% <89%> (+<1%)	⬆️

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[antonfirsov]

Target 2, then 1.

This means that it would be of key importance to validate the API with a simple GUI app that truly challenges the rendering engine! I just made this thing with Claude*. We need an equivalent C# app. It can of course do something else (as long as it does mass text rendering!), but we need to make sure we have an HTML5 reference app, and that its' functionality is kept in sync with our C# app's, so we can compare them.

the WebGPU IDrawingBackend implementation is purely experimental

We don't need it to be production ready, but if we find performance bottlenecks, we should identify where is the root cause: suboptimal backend implementation VS the API shape VS our library code.

I can chime in into this work later, but not right now.

*disclaimer: didn't spend time on deslopification