Add PNG screenshot support

Saves client version, map, and player info in PNG text fields.
Merge remote-tracking branch 'spearmint/master'

Conflicts:
	code/client/cl_main.c
	code/client/client.h
	code/qcommon/common.c
	code/renderercommon/tr_image_png.c
	code/renderercommon/tr_public.h
	code/renderergl1/tr_init.c
	code/renderergl1/tr_local.h