Added r_shadow_bouncegrid_lightpathsize which is an alternative to blur

Added r_shadow_bouncegrid_lightpathsize which is an alternative to blur
- but in practice this is just slower at a value of 3 than 1 is with
  blur, so blur remains the default approach.

Implemented r_shadow_bouncegrid_floatcolors cvar which uses fp16 and
fp32 float formats to increase dynamic range.

Overhauled the default values of the bouncegrid cvars to look better.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12277 d7cf8633-e32d-0410-b094-e92efae38249