Blender Git Loki

Fix: Image node alpha socket converted to operations twice

On selecting a multi-layer image with a combined pass, a "Combined"
socket is created and default combined pass socket "Image" is
disabled by setting `SOCK_UNAVAIL` flag. When converting into
operations, `ImageNode` converts alpha socket on finding any socket with
a combined pass without checking the flag.

Since commit rB93e2491ee724 an assertion fails when mapping sockets
twice because now map `add_new` is used.

Reviewed By: jbakker

Differential Revision: https://developer.blender.org/D11566