Blender Git Loki

Fix T48466: Multiple passes starting with 'Diffuse' in EXR file breaks its loading in Blender.

Issue here is that for Blender, any pass which name starts with 'Diffuse' is considered
a diffuse pass - and it does not really support several passes of the same type in renderresult.

So for now, passtype_from_name() is now checking whether a pass of same type already exists
in render layers, and return 0 (uknown passtype) in this case.