[canvaskit] Various cleanups around docs/build

 - Break up example into two pages
 - Deprecate SkCanvas.flush()
 - add build flag that helps with certain GPU's webgl support

Bug: skia:
Change-Id: I49b1dbfecc10265a14ef4cbe54f7e8f0306e12ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201649
Reviewed-by: Nathaniel Nifong <nifong@google.com>
5 files changed