Blender Git Loki

Bugfix #5333

Actually no real bug, but unfinished work in Composite:
The ZCombine node only accepted images, no value inputs for sockets.
Now only the first (top) input socket should be an image, the other ones
can use constant color or values too. Nice way to split an image in
foreground and background.

Also: added an output socket for the resulting Z value.

Do note that zcombone is aliased, no AA or sample info is available.
That's for another time.