Resolution scale with upscaling

[rocksdanister]

Rendering the shader at lower resolution and upscaling is a good way to improve performance, particularly useful with certain shaders that do use texture when it has minimal visual impact.

I am still learning Skia, I have tried:

1. Copying the lower resolution output to a offscreen canvas and upscaling which does work but very poor performance at higher shader resolution.
2. ViewBox - renders at full resolution.
3. effect.ToShader(uniforms, children, SKMatrix.CreateScale(scaleFactor, scaleFactor)) does not work.
4. canvas.Scale(scaleFactor, scaleFactor); renders at lower resolution but not sure how to upscale again without having the shader run at full resolution.
5. Setting ShaderWidth and Height and then adjusting stretch value - renders at full resolution.

Thanks for this project, it helped me finish the port of my weather app to Avalonia. The Windows version uses UWP with library ComputeSharp - one feature it has is the ability to set Resolution Scale (0 - 1).. I was hoping we can figure out something similar.