Save and restore execution mask when invoking children.

When invoking a child, they may overwrite the contents of dst.rgba.
This is where we store our execution masks, so it'd ruin the program's
state. We now store dst.rgba on the stack when children are running.

Change-Id: I557bf299000b7c41badb302c25807073b73daa3b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/637056
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
3 files changed