hidpi: Rescan monitors when screen scale factor changes

hidpi: Rescan monitors when screen scale factor changes

All the code in e.g. init_randr15() divides the physical resolutions with
the screen scale, however if we get the screen scale from xsettings
rather than e.g. GDK_SCALE the initial setup is using the wrong value.

So, whenever the screen scale size is changed we need to trigger
a re-read of the randr data