Orange: Further cleanup of EXR saving

Orange: Further cleanup of EXR saving

- F10 scene buttons now has options "half" and "zbuf" for exr saving.
  Note: when no float buffer is available, it always saves as "half",
  that's sufficient anyway, since half is 16 bits per channel.

- EXR in imbuf now uses compliant ibuf->ftype flags for denoting exr
  extensions such as 'half' and 'compression'.

- Removed ugly blenkernel dependency from exr module