Use sampling for bitmap shaders

Change-Id: Ifa656ffc83a1adb5670cff1e2ce77f2edb1d6b03
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/343776
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
8 files changed