Blender Git Loki

Partial revert of r46542.
Displaying image layers with flattened names (render_layer.render_pass) can lead to extremely long socket lists, especially with older files that were used with distinct render layers before. In general the flattening of names would be preferable, but it needs a sensible way of limiting the number of displayed outputs. For now the render layer dropdown selection is the best option to ensure usability.