Basic usage of blend_mode

Areas - fire and party alarms look better
Druggy 420 blazeit screen looks better
The whiteness of a flash looks nicer. Mob code makes me hesitant to really change this.
The nearsighted/welding mask dither looks better

If you want screenshots just ask.
Merge pull request #1729 from MrPerson/zone_of_the_blenders

Basic usage of blend_mode
Merge branch '501' of https://github.com/tgstation/-tg-station into BranchThatFucksRobRichardsUp

Conflicts:
	icons/mob/screen_gen.dmi

Really need to keep this branch up-to-date.