Refactored r_shadow_bouncegrid code into several functions, changed

Refactored r_shadow_bouncegrid code into several functions, changed
photon distribution math to make the photons more stable from frame to
frame (in dynamic mode), the number of photons fired is now based on
r_shadow_bouncegrid_intensityperphoton rather than always going for
maximum photons (accordingly r_shadow_bouncegrid_photons has been
renamed maxphotons and will still apply the old logic if it must).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12274 d7cf8633-e32d-0410-b094-e92efae38249