Blender Git Loki

Further tweak of Vector Blur and alpha;

- it now correctly pre-multiplies with alpha the RGB values for the
antialised mask (alpha artefacts were visible)
- The transparent layer will add speed vectors on top of the solid layer,
cancelling out cases where the solid layer was not moving (like in its
own antialising.
This works fine, for as long you don't render in a single pass trans-
parent faces that move on top of not-moving solid faces.