gpu impl for compose and combine maskfilters

remove "warning" in GrProcessorSet about coverage-as-alpha (we think the current behavior is correct)

update gpudevice::drawImage to check for maskfilter before trying to create its mask

Bug: skia:
Change-Id: I9ecb6cd25dd003bc19fa1e33edf6614a5ba4acb7
Reviewed-on: https://skia-review.googlesource.com/103761
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
6 files changed