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>
diff --git a/gm/imagefilters.cpp b/gm/imagefilters.cpp
index 8855afd..dd6287b 100644
--- a/gm/imagefilters.cpp
+++ b/gm/imagefilters.cpp
@@ -123,7 +123,7 @@
         canvas->save();
         SkRRect rr = SkRRect::MakeRectXY(r.makeOffset(dx, dy), 20, 20);
         canvas->clipRRect(rr, true);
-        canvas->saveLayer({ &rr.getBounds(), nullptr, filters[i].get(), 0 });
+        canvas->saveLayer({ &rr.getBounds(), nullptr, filters[i].get(), nullptr, nullptr, 0 });
         canvas->drawColor(0x40FFFFFF);
         canvas->restore();
         canvas->restore();