[sksg] Clear the context mask filter when applied via layers

Fixes rendering issues when a mask filter is applied to non-atomic
draws.

TBR=
Change-Id: I29486e7e759170bb2ece1e257bd6b981c161b1dc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222459
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed