Codechange: allow certain enumeration to be added

Otherwise C++20 doesn't like it.

Remove: ZOOM_LVL_COUNT

This is the only enumeration with a COUNT and END. The logic of the COUNT
implied that BEGIN could be non-zero, but all but two uses of zoom level
assume that BEGIN is zero, making the separate count only confusing.

Feature: Variable GUI scale.

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

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.

Feature: Add option to adjust font size separately from GUI size. (#7003)

Adds an option in the "Game Options" next to "Interface Size" called "Font Size". Available options are normal, double, and quad.