Fix #35703, Viewer node doesn't updates image sometimes. The viewer node was not getting correctly initialized node->id pointer to the Viewer image (thanks to Sergey for figuring that out). The original
proposal was to add another special init hack for the viewer node->id, but rather would do it right and so moved all the special init hacks for constant ID backpointers (Scene for RenderLayer, Composite,
Defocus, FileOutput and MovieClip for MovieClip, MovieDistortion and Stabilization nodes). These are now part of the local init callbacks functions of the appropriate nodes, using the new initfunc_api
callback which takes a Context pointer, so they have access to Scene.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57410 954f8c5b-7b00-dc11-b283-0030488c597c