Use a "decal" domain for filterMaskGPU

In addition to requiring absClear, the old approach yielded
asymmetrical blurs where the top and left edges got a "clamp" mode,
and the right and bottom edges effectively got a "decal" mode.

Bug: skia:
Change-Id: If72da4374eb2f0475acae59e269d44cb72c5ce95
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240759
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
5 files changed