Tweaks to movie clip prefetching after discussion with Sebastian

Don't use sequencer's Frames Prefetch option and rather fill the
whole cache with frames.

Since movie clip is not "classical" prefetcher (it doesn't do
anything while playing back) and main purpose is to speed up
reading for further tracking and so, such a change seems logical.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55459 954f8c5b-7b00-dc11-b283-0030488c597c