Added alpha output to movie clip compositor node

Pretty much straightforward change, made in the same way as
texture input node.

Shall not be any regressions or crashes when mixing usage
of 2.66 and current trunk.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55402 954f8c5b-7b00-dc11-b283-0030488c597c