Task scheduler ported form CYcles to C

Replaces ThreadedWorker and is gonna to be used
for threaded object update in the future and
some more upcoming changes.

But in general, it's to be used for any task
based subsystem in Blender.

Originally written by Brecht, with some fixes
and tweaks by self.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60710 954f8c5b-7b00-dc11-b283-0030488c597c
Option to overlay mask over the footage

Currently supports only two modes:
- Show alpha channel of the mask
- Multiply footage by the mask, which will give
  you final-looking combined image.

TODO: Currently rasterization happens on every
      redraw, need to cache rasterized mask
      somewhere to make redraw more realtime.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60711 954f8c5b-7b00-dc11-b283-0030488c597c