reworked decal system to limit the number of decal splats active at once

reworked decal system to limit the number of decal splats active at once
rather than simply allowing things to timeout, this does not directly
limit the number of triangles (because decal events on models may
produce hundreds or thousands of triangles) but it does reduce
worst-case performance issues
implemented compacting code for the new decal system, so there are no
wasted triangles when decals expire

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9507 d7cf8633-e32d-0410-b094-e92efae38249