- 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