[host] dxgi: decouple backends from the DXGI main struct #2363

Job	Run time
host-windows-cross	1m 4s
host-linux	30s
docs	36s
client (gcc, g++, xdg-shell, Release)	1m 27s
host-windows-native	1m 59s
client (gcc, g++, xdg-shell, Debug)	50s
module	24s
client (gcc, g++, libdecor, Release)	2m 37s
obs (gcc)	32s
client (gcc, g++, libdecor, Debug)	1m 3s
obs (clang)	33s
client (clang, clang++, xdg-shell, Release)	1m 34s
client (clang, clang++, xdg-shell, Debug)	1m 23s
client (clang, clang++, libdecor, Release)	1m 53s
client (clang, clang++, libdecor, Debug)	1m 14s
	17m 39s

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