Broken scaling with gtk,gl=on on a hidpi display

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1799792
Reported by 'Sebastian Krzyszkowiak' on 2018-10-24 :

Tested on QEMU 3.0.0 on Arch Linux.

I'm using a hidpi screen, and therefore use those environment variables in
order to have GTK+ apps properly scaled:

GDK_SCALE=2
GDK_DPI_SCALE=0.5

However, QEMU, when launched with "-display gtk,gl=on" option, doesn't
scale the window content properly, as seen on the attached screenshot.

Switching to "-display gtk,gl=off" and "-display sdl,gl=on" makes it work
fine.