Orange branch: OpenEXR finally in Blender!

Orange branch: OpenEXR finally in Blender!

Credits go to Gernot Ziegler, who originally coded EXR support, and to
Austin  Benesh for bringing it further. Kent Mein provided a lot of code
for integrating float buffers in Blender imbuf and ImBuf API cleanup,
and provided Make and Scons and static linking.

At this moment; the EXR libraries are a *dependency*, so you cannot get
the Orange branch compiled without having OpenEXR installed. Get the
(precompiled or sources) stuff from www.openexr.com. Current default is
that the headers and lib resides in /user/local/

Several changes/additions/fixes were added:

- EXR code only supported 'half' format (16 bits per channel). I've added
  float writing, but for reading it I need tomorrow. :)
- Quite some clumsy copying of data happened in EXR code.
- cleaned up the api calls already a bit, preparing for more advanced
  support
- Zbuffers were saved 16 bits, now 32 bits
- automatic adding of .exr extensions went wrong

Imbuf:

- added proper imbuf->flags and imbuf->mall support for float buffers, it
  was created for *each* imbuf. :)
- found bugs for float buffers in scaling and flipping. Code there will
  need more checks still
- imbuf also needs to be verified to behave properly when no 32 bits
  rect exists (for saving for example)

TODO:

- support internal float images for textures, backbuf, AO probes, and
  display in Image window

Hope this commit won't screwup syncing with bf-blender... :/