bagl: 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

Change-Id: I271fd87ccca4b6574d1b018a00f41b673d0f4211

Conflicts:

	opengl/libagl/Android.mk