qemu running slow when the window is in background

Reported by <jedinix> on IRC:

QEMU almost freezes when running with `GDK_BACKEND=x11` set and the parameter `gl=on` added to the `-display` option.

GDK_BACKEND=x11 qemu-system-x86_64 -nodefaults -no-user-config -enable-kvm -machine q35 -cpu host -m 4G -display gtk,gl=on -vga std -usb -device usb-kbd -drive file=/tmp/Win10.qcow2,media=disk,format=qcow2 -drive file=~/Downloads/Win10_2004_EnglishInternational_x64.iso,media=cdrom

Leaving out `GDK_BACKEND=x11` or `gl=on` fixes the issue.