Blender Git Loki

Clip editor prefetching changes

Now prefetching happens from cursor up to scene end frame
and then from cursor down to scene start frame.

Also disabled prefetching for non-unidistorted proxies
because of issues with timecodes (seek horribly fails
in some cases). This seek need to be fixed since it's
actually happens for manual timeline scrubbing as well,
but that's another story..