Also, check some expected conditions at parse time instead of somewhereduring use of the data.

Previously, these were both typed as `int`, which means you could mixand match them erroneously. While the character code can't be made adistinct type (because it's used for `chr`/`ord`), typing glyph indicesas a distinct type means these can't be fully swapped.Unfortunately, you can still go back to the base type, so glyph indicesstill work as character codes. But this is still sufficient to catcherrors such as the wrong call to `FT2Font.get_kerning` in`_mathtext.py`.

The `FontInfo.num` value returned by `TruetypeFonts._get_info` is acharacter code, but `FT2Font.get_kerning` takes *glyph indices*, meaningthat kerning was likely off in most cases.