Merge freetype text plugin

This merges paibox's freetype-based text plugin.  Works quite nicely and seems to be quite efficient.  Instead of drawing to one big giant texture, it caches the glyphs individually and draws each character, which is much better than the way that OBS1 handled it (which was by drawing it all using GDI on to one big texture, which could get cut off if it became too big).

I modified the plugin to look up system fonts and use system fonts instead of having to force the user to specify the font file.  Getting system fonts working with freetype was excruciatingly difficult due to a variety of reasons, but ended up being doable.

(Ah, and as I write this I just remembered I forgot to account for bitmap fonts, so I'll have to fix that before merging)