Blender Git Loki

Z sampling for nodes backdrop
As it turned out, the node space was the only place we have the option to sample the backdrop buffer RGB, but not the zed.
This code is copied from image space.

Now one can use the viewer node and sample the depth while looking at the color buffer (so Map Range Node works even better)
Patch written during BlenderPRO 2012, Brasília ;)
(reviewed by Lukas Toenne)