multiview: support for images output from composite

This is not the nicest solution, but it's working.

Note:
When I save the file (F3 from Image Editor) it's never saving the compo. I'll look at that.
error: conflicts with new declaration with C linkage
error: ISO C90 forbids mixed declarations and code
error: ISO C90 forbids mixed declarations and code
Merge branch 'multiview' of github.com:dfelinto/blender into multiview