Force movie clips always use default alpha mode, before this straight
alpha was used for them which doesn't work for cleaned footage stored
in EXR file format.

Perhaps we need to support configurable alpha mode for clips, but
that's for later (maybe even after release),

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54640 954f8c5b-7b00-dc11-b283-0030488c597c