Add a custom ui scale for HIDPI displays

[Bartandroid123]

As stated above, add a custom ui scale slider so the player can properly scale to small HIDPI/retina displays

[waltmck]

This is something that is usually handled through GTK. On Wayland, fractional scaling will "just work" through the fractional scaling protocol.

If you are on X, you should be able to adjust scaling factor with the $GDK_SCALE environment variable (same as any GTK application).

Either way, this feature request should definitely not be implemented.