Using output nodes inside node groups in compositor node trees doesn't
work for the realtime compositor.
Currently, the realtime compositor only considers top level output
nodes. That means if a user edits a node group and adds an output node
in the group, the output node outside of the node group will still be
used, which breaks the temporary viewers workflow where users debug
results inside a node group.
This patch fixes that by first considering the output nodes in the
active context, then consider the root context as a fallback. This is
mostly consistent with the CPU compositor, but the realtime compositor
allow viewing node group output nodes even if no output nodes exist at
the top level context.
Differential Revision: https://developer.blender.org/D16446
Reviewed By: Clement Foucault
8ef092d2d8