decals added back due to popular demand, currently not at all optimized (they're...

decals added back due to popular demand, currently not at all optimized (they're just particles) and not clipped to surfaces
cl_decals 1 to enable (this is saved to config), cl_decals_time and cl_decals_fadetime change settings for them (these are also saved to config)
redesigned particle additive flag to be a blend mode number (PBLEND_ALPHA, PBLEND_ADD, or PBLEND_MOD - for decals), lots of special hacks for the nifty new PBLEND_MOD mode

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@2659 d7cf8633-e32d-0410-b094-e92efae38249