meta-monitor-manager-xrandr.c: screen size fixes #103

Job	Run time
build / build (mint22, linuxmintd/mint22-amd64, Mint 22, true) / Mint 22	5m 31s
build / build (lmde6, linuxmintd/lmde6-amd64, LMDE 6, false) / LMDE 6	5m 20s
	10m 51s

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