Unlike other shape path effects, merge paths disables the rendering of
any preceding paints - it only extracts the merged geometry from the
stack.
Update the shape layer attacher logic to suppress paints under merge
paths.
TBR=
Change-Id: I414134839de9eaa4b0f828d8dc6d4721620242bb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300897
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
