Add SKSL editor and another canvas backed by offscreen gpu canvas

[BandarB7]

• SkiaSharpFiddle now has 2 text editors and 2 canvasas on UI
• All drawing commands are executed through 1-cpu canvas
• Result of cpu canvas is copied to gpu canvas where SKSL is executed and copied back to cpu canvas
• GlContexts is copied from SkiaSharp unit tests

This is a workaround to enable SKSL to work using SkiaSharp 2.XX on WPF. In SkiaSharp 3 (pre-release), this approach is not needed