GdipPrivateAddFontFile broken on some platforms

[directhex]

Under circumstances I haven't been able to pin down, creating a font collection and adding a font from disk to it does not result in a workable font.

In CI, this results in tests/testfont.c failing on GdipCreateFontFamilyFromName (CodeNewRoman, collection, &family); with FontFamilyNotFound on my Macbook but passing fine on my Ubuntu desktop.

This behaviour looks a LOT like the behaviour reported in dotnet/runtime#26460 - and, indeed, the user's test code passes fine on my Ubuntu deskop but fails on my Macbook.