PangoWin32: Improve thread safety

Hi,

This is an attempt to improve PangoWin32's thread-safety in terms of font discovery, which should now be simplified as we are using HarfBuzz for shaping for all platforms. We might be able hope to still try to do better to go without the Pango warnings that pop up when we are using some fallback or when fallbacks could not be found, but at least now the test-pangocairo-threads test program passes. I will try to look further into this.

I test ran 'meson test test-pangocairo-threads' for 30 successive runs, which passed.

With blessings, thank you!

p.s. For starters, as the first commit, I also decided to fix up the code formatting of pangowin32-fontmap.c by getting rid of the tabs and correcting some formatting issues here and there.