Assorted bugfixes for the non-analytical mask code path.
1) SkSG modulatePaint() should only override the blend mode when
one is specified (!= kSrcOver).
2) Some modes (notably intersect) require touching pixels outside the
mask draw geometry. These modes must be applied as a layer.
Introduce an explicit layer node in SkSG, and inject for masks which
require it.
Also refactor Subtract to use more natural blend and pathops modes,
instead of always inverting geometry.
TBR=
Change-Id: I412168d1ff61eb8e59907babe8f0e091f6fffacf
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303997
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
