WIP: DPI scaling improvements

[whitequark]

This PR includes three parts:

1. Splitting GetContentScale() into GetUIScale() and GetFontScale()
2. Making the two possible to override without recompiling ngscopeclient, on any platform, using NGSCOPECLIENT_UI_SCALE and NGSCOPECLIENT_FONT_SCALE
3. Auto-detecting these using GDK_SCALE and GDK_DPI_SCALE on Linux, which should cover both GTK and Qt

This probably breaks scaling, to the extent it did work, on Windows and macOS.