[host] d12: make RGB24 optional, disabled by default #2452

Job	Run time
docs	38s
host-windows-cross	38s
client (gcc, g++, xdg-shell, Release)	1m 24s
host-linux	50s
client (gcc, g++, xdg-shell, Debug)	42s
host-windows-native	1m 31s
module	14s
client (gcc, g++, libdecor, Release)	1m 14s
obs (gcc)	22s
obs (clang)	28s
client (gcc, g++, libdecor, Debug)	1m 8s
client (clang, clang++, xdg-shell, Release)	1m 12s
client (clang, clang++, xdg-shell, Debug)	58s
client (clang, clang++, libdecor, Release)	1m 40s
client (clang, clang++, libdecor, Debug)	1m 5s
	14m 4s

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