SF: Fix invalid reduction of transparent region from a layer.

Reduce transparent region from a layer only if the transparent
region has valid intersection with the layer. Otherwise, reducing
a transparent region lying completely outside of the layer leads to
invalid sourcecrop values for the layer.

Change-Id: Iee9ba53a8072c21c9ba8a6d58cb14ca30f8600ba
CRs-Fixed: 620973