Feature: Variable GUI scale.

GUI scale is now variable from 100% to 500%, and no longer restricted to
powers-of-2.

Change: Use power-of-2 scaling for some dimensions in GUI.

These are related to drawing sprites that are normally drawn on the
landscape, and should therefore still follow power-of-2 scaling to fit
correctly.

Fix #9995: Adjust viewport zoom level for HiDPI displays

On HiDPI screens the zoom level is increased for detailed rendering. This causes hard-coded zoom levels to be off by this adjustment. To fix these default zoom levels, we scale the zoom level based on `_gui_zoom` to get the scaled zoom level.