Implement radial wipe with a sweep gradient shader mask filter.
The implementation is slightly convoluted because edge feathering requires a real blur, which in turn requires content layer isolation.
So there are two distinct operation modes:
- no feather -> draw the content directly into the dest buffer, with the mask filter
deferred in SG context
- feather -> draw the content into a separate layer, then blend (dstOut) the composed
blur+shader mask on top
Change-Id: I253701aff42db8010ce463762252c262e2c5d92b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222596
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
