Add SkRuntimeShaderBuilder, clean up SkRuntimeEffect API a bit

Utility class for getting named access to uniforms and children of an
SkRuntimeEffect (also functions as an example of using the
SkRuntimeEffect public API).

Moved several internal SkRuntimeEffect functions to private, and added
findInput/findChild helpers.

Change-Id: I8c2e7745ea81670a49b7ab2f51ce44a8d8169278
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/286516
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
4 files changed