Blender Git Loki

Render API: Allow renderers to allocate additional passes

This commit adds the add_pass function to the renderer API, which allows a renderer
to add a pass to a certain or all render layers. Since the RNA only has 32bit integers as
argument types, the position of the passtype bit is passed instead of the actual passtype.
When additional RenderResults are allocated, all additional passes added to the main result
are added to it as well.