Skia: add support for CPU based rendering (optional)

[tronical]

trafficstars

Since Skia can render with just the CPU, it would be nice if we offered this as an option. This would likely still use a Skia backed GL/D3D/Metal surface, but it could be partially updated with the result of the CPU rendering.

[ogoffart]

Is that a compile-time or run-time setting?

[tronical]

I think similar to renderer-winit-skia-opengl this would be a crate feature to enable the compile-time support for it and when Skia is selected, that would be the method of rendering.

[tronical]

I realise that we could also offer this as a run-time setting, if we depend on softbuffer and simply fall back to it.