Runtime SkSL: API sketch for child shaders/effects

Only works on GPU backend for now, and still only supports
single-argument sample (at original coords).

Change-Id: I4c36ebd0c370ca65126786802c5ea268c3f32edd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/260899
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed