Support for painting of stereo images (no real depth painting yet)

Idea poll:
When in stereo mode you paint we paint to both buffers at the same time,
however the brush should get an offset parameter (pixel separation) that would
allow the user to paint in depth.

The brush 'icon' (the circle thing) could also be drawn with this
offset.

Bonus: make grease pencil (View mode) do the same thing. Basicaly we can
add a 'viewoffset' property to each line and use it at drawing time (no
idea of GPencil code, just speculating).