[sksg] Fix inval rect mapping

Return false from mapRect() doesn't mean the op failed.

TBR=
Change-Id: I0582fde3efaa792010f27e3684cfe9c4332e29dc
Reviewed-on: https://skia-review.googlesource.com/90021
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed