Add fragment shader effect support.

[kkwpsv]

Is your feature request related to a problem? Please describe.

I want to use fragment shader effects and animations in Avalonia, like ShaderEffect in WPF.

Describe the solution you'd like

Now, we mainly use Skia for rendering. It has SKSL, which can be used for shader effect.

Describe alternatives you've considered

Use GLSL or others. But it is difficult to integrate into the current rendering pipeline of Avalonia.
And, different underlying rendering methods like vulkan or opengl require the use of different rendering languages. This is not good for cross-platform.

Additional context

I have integrated the SKSL effect into Avalonia. I will submit a draft PR.
It requires a SkiaSharp change.
And I don't have a good design for Uniform-related APIs. I need some advice.

[kkwpsv]

Here is the draft PR.