Since rB329b4c3363e4 a texture preview is not rendered through the
preview.blend and a renderengine anymore [unlike materials etc.]
Alpha wasnt handled in that commit, take that into account now.
sidenote: not sure if we should be looking into drawing alpha with a
checkerboard in the background for texture previews (see texture
sidenote 2: might also be good to document where the "calculate" and
"invert" alpha options are still used? [looks a bit inconsistent:
compositor uses it, Image Editor as well as Render Engines dont?]