Made some improvements to the point density texture. Added support
for tweaking the falloff with a custom curve. Also coded new
falloff types based on the age or velocity of particles.
Also added a test break check to the volumetric shade cache code,
to avoid nasty hangups from the preview render (on render, exit,
etc).
ui/ --> startup/bl_ui
op/ --> startup/bl_operators
scripts/startup/ is now the only auto-loading script dir which gives some speedup for blender loading too.
~/.blender/2.56/scripts/startup works for auto-loading scripts too.
