This makes GtkSettings values on X11 match what we get on
other backends.

Reporting size settings in logical pixels (i.e for scale
== 1) is useful for properly supporting mixed-DPI setups.
As X11 doesn't support mixed-DPI setups anyway, XSettings
doesn't bother providing logical values. Thus we scale
from physical to logical values ourselves.

Fixes #5223
Fixes #5230