-display option not propagated to gtk3 platformtheme

When the DISPLAY environment variable and the -display command-line options differ from eachother, the libqgtk3.so platformtheme may crash, like so:

 $ DISPLAY= QT_QPA_PLATFORMTHEME=gtk3 qdbusviewer -display :0

(qdbusviewer:8735): Gtk-WARNING **: 12:03:09.078: cannot open display: 

This affects the behavior of applications like gpg-agent, that unset the DISPLAY variable, and expect a subprocess to work with just passing the `-display` option. As such, this can also be reproduced with `DISPLAY= QT_QPA_PLATFORMTHEME=gtk3 pinentry-qt --display :0`.

Note that this probably isn't an issue for wayland thanks to the absence of an equivalent -display option. Unless I'm wrong about this, that is...