Enforce runtime effect's isOpaque on the GPU backend

CPU already does this. Forcing it this way ensures that optimizations
related to opaqueness are correct.

Change-Id: Iaec3990ec0102b4c11f942774361239d3583adfd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369323
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed