Blender Git Loki

* A few volume rendering tweaks:

- modified point density so that it returns a more consistent
density with regards to search radius. Previously larger radii
would give much higher density but this is equalised out now.

- Added a new volume material option 'density scale'. This is an
overall scale multiplier for density, allowing you to (for
example) crank down the density to a more desirable range if
you're working at a large physical scale. Volume rendering is
fundamentally scale dependant so this lets you correct to get the
right visual result.

- Also tweaked a few constants, old files won't render exactly
the same, just minor things though.