This change also remove the restrictions of operations on
graphicscommand.Image. For example, now DrawTriangles and
ReplacePixels can be mixed on the same image.
Fixes#1022
After this change, GL functions for mobiles, especially Android, are
native ones instead of golang.org/x/mobile/gl functions in order to
reduce goroutine context switches.
On gomobile-build, golang.org/x/mobile/gl functions are still used.
Fixes#1387
