revert r53570, alpha background is useful for projection paint 'Quick Edit',
it would also give odd results drawing semi transparent faces will give partial alpha which is strange to have with solid backdrop.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53579 954f8c5b-7b00-dc11-b283-0030488c597c