Fix for #34739 and #35060, avoid ambiguity in compositor viewer nodes.

The design changes coming with pynodes for the node editor allow editing multiple node groups or pinning. This is great for working on different node groups without switching between them all the time, but it causes a problem for viewer nodes: these nodes all write to the same Image data by design, causing access conflicts and in some cases memory corruption. This was not a problem before pynodes because the editor would only allow 1 edited node group at any time. With the new flexibility of node editors this restriction is gone.

In order to avoid concurrent write access to the viewer image buffer and resolve the ambiguity this patch adds an "active viewer key" to the scene->nodetree (added in bNodeTree instead of Scene due to otherwise circular DNA includes). This key identifies a specific node tree/group instance, which enables the compositor to selectively enable only 1 viewer node.

The active viewer key is switched when opening/closing node groups (push/pop on the snode->treepath stack) or when selecting a viewer node. This way only the "last edited" viewer will be active.

Eventually it would be nicer if each viewer had its own buffer per node space so one could actually compare viewers without switching. But that is a major redesign of viewer nodes and images, not a quick fix for bcon4 ...

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56271 954f8c5b-7b00-dc11-b283-0030488c597c