libagl: Allow use of copybit to accelerate 2D rendering

This improves performance on devices that uses copybit.
Enable with TARGET_LIBAGL_USE_GRALLOC_COPYBITS := true

Forward ported from Gingerbread

Conflicts:
	include/ui/Region.h

Change-Id: If69c6f0580c8f37cabda20da2cce39714addb669