Fast path Region::orSelf when empty

If a Region is empty, orSelf is equivalent to just assigning from the
other Region/Rect, so we don't need to perform a whole rasterization
pass.

Test: libsurfaceflinger_unittest
Test: simpleperf measurement of SF shows workload reduction
Bug: 153112939
Change-Id: Iefa0e0177a0e85ce72e37842885d3c8b68ce6025