Avoid blur behind for layers that are fully opaque.

This addresses cases where the layer is a solid color or has an
opaque buffer that will result in the blur being completely
overwritten by the layer's contents.

Bug: 188050128
Test: atest librenderengine_test and verfied by mskp capture
Change-Id: I5ef76eca9a3dda21d1faa652da60abb22282925a