Expose unitsPerEm

[dpwiz]

I just found some font (noto emoji) giving advances way too wide and they happen to use unitsPerEm=2600 instead of vanilla 1000.

Unless I'm missing something right now we must dive into font-blob-head table to read the correct value.

I'm not sure if that should go into FontInfo or be a standalone function... Maybe write it down right next to the gpos block in the glyph to avoid doing lookups later.

[JimmyLefevre]

I am planning on extending the API soon, and some kind of complement to kbts_GetFontInfo that adds metrics is definitely planned.

It doesn't exist right now because I added the OS2 table pretty late, and because I haven't needed it yet, but there's no reason for it not to be there.