user32: fix stale sysfont values upon locale switch
If a prefix is created in one locale, then a program is run in it in a
different locale, most locale-dependent font settings are already reset,
yet there are a few values that aren't, namely those stored in
'HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics' *Font keys.
This might mess up different things, as one of the fields stores
charset, which is a base for certain MultiByteToWideChar conversions.
This is the cleanest solution I could come up with.