This patch cleans up the sequencer core by replacing the caching system
(TStripElems) with a hash based system, which is:
a) a lot faster
b) a lot more readable
c) a lot more memory conserving
The new caching system is also a good building ground for
a) sub frame precision rendering (even on scene strips)
b) multi core rendering (threaded rendering is still disabled, but can
be extended now to arbitrary core numbers)
I tested the code on an extensive editing session today and had no
crashes during 4 hours of editing. So I consider it very stable.
