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 'master' of https://github.com/Giacom/-tg-station into 501

Conflicts:
	code/_onclick/hud/hud.dm
	code/_onclick/hud/human.dm
	code/modules/mob/living/carbon/human/life.dm
	icons/mob/screen_gen.dmi
Merge branch 'master' of https://github.com/tgstation/-tg-station into 501
Fixed the .dme
Merge pull request #2374 from tgstation/501

Merging 501 with Master.