New cvars:
cl_cshift_bonus: Set to 0 to disable bonus flashes.
cl_cshift_contents: Set to 0 to disable content blends.
cl_cshift_damage: Set to 0 to disable damage blends.
cl_cshift_powerup: Set to 0 to disable powerup (quad, pent, etc) blends.

All of these cvars work for both GL and software, and all of them check
the value of the "cshifts" serverinfo. cshifts is a bit field, with the
following bits defined:

bonus: 1
contents: 2
damage: 4
powerup: 8

An admin can force any of these to be respected by choosing the numbers of
the cshifts to be enforced, and adding them up.

examples:

serverinfo cshifts 15

turns them all on.

serverinfo cshifts 10

turns on powerup and contents shifts.