If using the same texture to store the image and mask data (for
example when using texture atlas) the mask and image (RGB) coords
aren't the same anymore. With this commit we always define the mask
coords separately.

(cherry picked from commit bdce4e29)

Conflicts:
	vcl/opengl/program.cxx

Change-Id: Ie33f87a6e9ab398972c6a3d5938e5f1364c82d36
Reviewed-on: https://gerrit.libreoffice.org/18012Reviewed-by: Tor Lillqvist <tml@collabora.com>
Tested-by: Tor Lillqvist <tml@collabora.com>