Blender Git Loki

- Restored "dither" option for conversion from float -> byte images.
This now is a post-process option only (used to be in render).
It is only handled within the Imbuf/ module, on conversions from float
to byte rect, which atm mostly happens on saving images.

- Small fix: when using Scene RenderLayer nodes, the speed vectors for
these nodes were not created when that scene had "Do Composite" off.