Introduce sksg::BlendModeEffect and use to wrap Skottie layers based on
their "bm" property.
Depending on the presence of non-trivial layer blend modes and the nature
of the destination buffer (fully transparent vs. unknown/pre-filled), we
may now need to render animation frames into a separate layer for correct
compositing.
Track the presence of non-trivial layer blend modes such that we only incur
this extra layer overhead when needed. Also allow clients to pass a "drawing
to fully transparent buffer" hint such that we can avoid the extra layer even
when blend modes are present.
Change-Id: Iaf645878666da4349d0bef8890bbecad23a0aa9b
Reviewed-on: https://skia-review.googlesource.com/c/194840
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
bd64f18947