implement SkColorFilterShader::isOpaque()

Change-Id: I53872aa96b9cfcb3352cf8d48329e32f19cb97f8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252789
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
2 files changed