This implements basic color grading modifiers in sequencer, supporting
color balance, RGB curves and HUE corrections.
Implementation is close to object modifiers, some details are there:
http://wiki.blender.org/index.php/User:Nazg-gul/SequencerModifiers
Modifiers supports multi-threaded calculation, masks and instant
parameter changes.
Also added cache for pre-processed image buffers for current frame,
so changing sequence properties does not require rendering of original
sequence (like rendering scene, loading file from disk and so)
