[skottie] Fix 'subtract' masks

Empirically, AE applies subtract masks as inverted intersect masks: it
applies the opacity *outside* the mask geometry.

Adjust the masking logic accordingly.

TBR=
Change-Id: If8d9dedbed9ce01b623b6c86ea91e494823d5dc5
Reviewed-on: https://skia-review.googlesource.com/138580
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed