[WIP] Skia SKPaint pooling

[MarchingCube]

What does the pull request do?

Leverages my recent changes to SkiaSharp (mono/SkiaSharp#807) to speed up rendering by avoiding constant allocation of SKPaint objects. I benchmarked this by rendering a Rectangle 1000 times using immediate renderer.

SkiaSharp did not push any new package yet, so this will have to wait until they release a new version on nuget.

What is the current behavior?

No pooling

Method	Mean	Error	StdDev	Gen 0/1k Op	Gen 1/1k Op	Gen 2/1k Op	Allocated Memory/Op
RenderRectangle	11.34 ms	0.0726 ms	0.0606 ms	2718.7500	-	-	2.05 MB

Additionally ThreadSafeObjectPool has a locking issue - Get and Return obtain 2 different locks which can lead to some strange race conditions.

What is the updated/expected behavior with this PR?

Pooling

Method	Mean	Error	StdDev	Gen 0/1k Op	Gen 1/1k Op	Gen 2/1k Op	Allocated Memory/Op
RenderRectangle	10.08 ms	0.0190 ms	0.0168 ms	2437.5000	-	-	1.84 MB

How was the solution implemented (if it's not obvious)?

DrawingContextImpl.CreatePaint will utilize one of three available SKPaint pools. If we are dealing with most common case (solid color brushes) it will use either a pool for normal brushes or use a pen pool (that enables stroke). In case of other brush type the General pool will be used that supports any kind of paint. Pools for brushes and pens do not reset, so it is important to ensure all paint properties will be assigned during construction (Color for brushes, all stroke properties for pens).

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation

Breaking changes

Fixed issues

cc: @Gillibald @kekekeks

[grokys]

@Gillibald @kekekeks any thoughts on this? LGTM from a quick review but I'm not so familiar with this code. Any idea why it's still WIP?

[Gillibald]

I think this is currently blocked by the SkiaSharp dependency. It uses a not yet released API. I like these changes a lot.

[Gillibald]

I thought about this a bit more. Maybe we should cache shaders instead of paint and mutate a single instance of paint instead. We could cache resources like shaders per DrawingContext. There are some situations where we need a new instance of paint but the common scenario just needs one per DrawingContext.

This approach would make it easier to control aliasing and interpolation. A draw call would not need an extra parameter for filter, interpretation, aliasing and instead we configure this on the DrawingContext's paint.

[MarchingCube]

I think this need a bit more thinking about. I think we still have cases where we multiple SKPaint instances. Newer Skia also made creating new objects a bit cheaper, so new profiling run is needed. I will close this for now.