Blender Git Loki

Orange: tested all EXR demo images from openexr.com, found two issues;

- images with a so-called "data window" (have negative start coordinate)
did not read correctly
- negative colors were not clamped yet in imbuf

Now there's still some compliancy issues with zbuffers... you can save
it either as unsigned int or as float, whilst blender renders zbuffer in
signed int. :)