As part of the fix for T51587, I removed the Depth output for non-Multilayer
images since it seemed weird that PNGs etc. that don't have a Z pass still get
a socket for it.
However, I forgot about non-multilayer EXRs, which are a special case that can
actually have a Z pass.
Therefore, this commit brings back the Depth output for non-multilayer images
just like it was in 2.78.
f2728939df