Sequencer: per-sequence modifier stack for color grading

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)

release/scripts/startup/bl_ui/space_image.py

@@ -774,7 +774,7 @@ class IMAGE_PT_paint_curve(BrushButtonsPanel, Panel):
         toolsettings = context.tool_settings.image_paint
         brush = toolsettings.brush
 
-        layout.template_curve_mapping(brush, "curve")
+        layout.template_curve_mapping(brush, "curve", type='COLOR')
 
         row = layout.row(align=True)
         row.operator("brush.curve_preset", icon='SMOOTHCURVE', text="").shape = 'SMOOTH'