Fix center of rotation with rotation_mode='anchor'

Glyph indices are specific to each font. It does not make sense to fall
back based on glyph index to another font.

This could only really be populated by calling `FT2Font.set_text`, but
even that was fragile. If a fallback font was used for a character with
the same glyph index as a previous character in the main font, then
these lookups could be overwritten to the fallback instead of the main
font, with a completely different character!

Fortunately, nothing actually uses or requires a fallback through glyph
indices.

Remove ttconv backwards-compatibility code

Remove fallback code for glyph indices