wayland windows don't rescale when display scale changes

A QT5 wayland window properly rescales when it is moved between displays with different scales, but fails to rescale in these display scale change cases:

• Display is unplugged, sending containing workspace to a different scale display
• Display is replugged, restoring containing workspace to a different scale display
• Display scale is changed by user

The user change case is easy to reproduce with any sway setup:

Find your display ID

swaymsg -t get_outputs

Set scale to 1

swaymsg output eDP-1 scale 1

Open a QT window

kdialog --sorry "qt doesn't catch scale changes"

Change display scale

swaymsg output eDP-1 scale 3

Downstream report:

https://github.com/qutebrowser/qutebrowser/issues/5026

Other factors/non-factors:

• GTK3 applications rescale in these cases.
• QT_AUTO_SCREEN_SCALE_FACTOR=1 doesn't change behavior
• happens with both QT_QPA_PLATFORM=wayland and wayland-egl