[canvaskit] Add _RTShaderFactory for demos

Change-Id: Ib6dc1ff861efe4040508fea09c8a66d8d06db597
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/258568
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
5 files changed