Better handing of frames below actual sequence range

When trying to load file below actual sequence range
(like trying to load file for scene frame 10 when clip's
start frame is 20) first frame from file sequence is used.

Before this change first file used to be loaded for every
scene frame below start frame, which polluted memory with
unwanted data.

Now first frame would be loaded only once in this case.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55570 954f8c5b-7b00-dc11-b283-0030488c597c
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..

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55571 954f8c5b-7b00-dc11-b283-0030488c597c