Fix kerning of mathtext

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

Author: QuLogic

Diff for: lib/matplotlib/_mathtext.py

@@ -423,7 +423,9 @@ def get_kern(self, font1: str, fontclass1: str, sym1: str, fontsize1: float,
             info1 = self._get_info(font1, fontclass1, sym1, fontsize1, dpi)
             info2 = self._get_info(font2, fontclass2, sym2, fontsize2, dpi)
             font = info1.font
-            return font.get_kerning(info1.num, info2.num, Kerning.DEFAULT) / 64
+            return font.get_kerning(font.get_char_index(info1.num),
+                                    font.get_char_index(info2.num),
+                                    Kerning.DEFAULT) / 64
         return super().get_kern(font1, fontclass1, sym1, fontsize1,
                                 font2, fontclass2, sym2, fontsize2, dpi)