Don't allow layers that draw via drawGL to merge with other surfaces

bug:6694807

They don't paint onto surfacebackings, and draw afterward, so ordering issues
occur. Ideally, they should still be allowed to merge onto surfaces and not
allow other painting layers to join the surface afterward, but this shouldn't be
wasteful in practice.

Change-Id: I7a8764bc2b8e531e718602c6114b027a9dbce42d
3 files changed