GdkWin32: don't force native font settings when using fontconfig renderer
Currently win32 font settings are tailored for the native DWrite/ClearType renderer.
It doesn't make sense to enforce them when using other renderers like fontconfig. Instead, let settings be customizable via Gtk's 'settings.ini'.
Note: This change doesn't affect default setups, only users opting for a different renderer (e.g. by setting PANGOCAIRO_BACKEND=fc
environment variable) will unlock the possibility to use custom font settings.
For reference, some exchanges on discourse.