Initial clip-mask-layer support

SkBitmapDevice-only implementation.

Will add A8 fast path specializations in a follow-up.

Change-Id: I2ccb1ffba3689e92ac90a23e94737471dfb121a1
BUG=skia:6005

Change-Id: I2ccb1ffba3689e92ac90a23e94737471dfb121a1
Reviewed-on: https://skia-review.googlesource.com/14183
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
30 files changed