Blender Git Loki

Multi-threaded frame calculation for movie clip proxies

This commit implements multi-threaded calculation of frames
when building proxies. Both scaling and undistortion steps
are now threaded.

Frames and proxy resolution are still handled one-by-one,
saving files after every single step. So if HDD is not so
fast, this commit could have not so much benefit.

Internal changes:

- Added IMB_scaleImBuf_threaded which scales given image
buffer in multiple threads and uses bilinear filtering.

- libmv's camera intrinsics now have SetThreads() method
which is used to specify how many OpenMP threads to use
for buffer distortion/undistortion.

And yeah, this code is using OpenMP for threading.

- Reshuffled a bit libmv-capi calls and added function
BKE_tracking_distortion_set_threads to specify number
of threads used by intrinscis.