Fix #34967: Display transform makes byte image with alpha=0 black

Skip premultiplication/de-premultiplication when acquiring display
buffer for a byte image. Will make conversion a bit faster also :)

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56065 954f8c5b-7b00-dc11-b283-0030488c597c