sf-caching: Skip hole punch for layer that blends

If the layer blends (i.e. BlendMode != NONE), we cannot punch a hole for
it, since we'll no longer see the content behind it.

We cannot check isOpaque, since a layer with rounded corners is not
consider opaque. (And we want to be able to hole punch for rounded
corners.)

Fixes: 221224727
Test: libcompositionengine_test
Change-Id: Ia8bb27ce78baf8159a60945e37634c053577b2dd