Fix #33747: do better backwards compatibility for image transparency changes.
The use alpha option moved from the texture datablock to the image, and now it
will duplicate the image datablock in case you have one texture using alpha and
the other not.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54392 954f8c5b-7b00-dc11-b283-0030488c597c