Bug fix #34174

Multilayer files saved from Maya (and I bet others) store the 'primary' layer without
layer or pass name, just as R G B A. Allows viewers to show stuff too, I guess.

Blender now reads this as well, just allowing an empty string for the layer and pass.



git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54412 954f8c5b-7b00-dc11-b283-0030488c597c