Fix for image transparency backwards compatibility. Now the texture datablock has
a Use Alpha option again. This makes the case where you enabled Premultiply on the
image and disabled Use Alpha on the texture work again.

That's mostly useful when you have a straight alpha image file which has no useful
RGB colors in zero alpha regions (e.g. renders). Then sometimes you don't want to
use the alpha for the texture stack mixing, but you still want to multiply it into
the RGB channels to avoid a blocky transition into zero alpha regions.

This also removes the version patch that copied image datablocks because it's not
reliable and might be causing bug #34434. This does mean we are no longer backwards
compatible for cases where two different texture datablocks with Use Alpha enabled
and disabled where using the same image.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55022 954f8c5b-7b00-dc11-b283-0030488c597c