change onProgram to return its color, or an invalid-color if failed

If we like this, will follow with a CL to change colorFilters to take
a color and return a color.

Change-Id: Ibd99d18dd7c21fa592500bf371d94a90a01f48b5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277062
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
16 files changed