This means that modifier would operate on buffer which was passed to it,
without creating copy of image buffer and operating on it.
All current modifiers fit into this model and if it would need to have
original buffer on modifier calculation, that particular modifier can
create copy.
Gives some percentage of boost.
Behaves in exactly the same way as bright/contrast compositor node.
Some code could be de-duplicated, like contrast formula and mask
influence, but wouldn't call it stopper for commit since it's
already needed for grading Mango.
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)
