Unify window size rounding strategy

[xStrom]

There has been some divergence in the rounding strategy of window sizes. This PR unifies the strategy on Windows and GTK.
(On macOS the OS controls it. I didn't look at X11/Wayland in much depth.)

Before this PR some places used ScaledArea::from_dp (Size::expand under the hood) and others used f64::round. Now all the window size scaling is done via ScaledArea::from_dp. A unified strategy will help cut down on obscure bugs. This will also allow for easier strategy changes in the future if there is a need, as we can just change that one method.

This PR also contains:

• A fix for get_size not being in display points on Windows (backport of glazier#35)
• An implementation of get_scale on macOS (backport of glazier#36)
• A few comment types converted from dobule- to triple-slash