Changes for recent image alpha rework.

Addons maintainers please double check the changes.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4137 96b784f8-4ae3-dd11-aff7-0030488c597c
Fix [#33759] Can't use UV image export UV_layout error

Another glitch from recent alpha work. Also modified povray's code. As usual, maintainders of these scripts, please double check changes are ok! :)

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4138 96b784f8-4ae3-dd11-aff7-0030488c597c