Fix center of rotation with rotation_mode='anchor'

Remove ttconv backwards-compatibility code

Remove fallback code for glyph indices

This allows checking that there are no _new_ failures, without
committing the new figures to the repo until the branch is complete.

ci: Preload existing test images from text-overhaul-figures branch

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

ci: Fix image preload with multiple conflicts

Add typing to AFM parser

Previously, this was supposed to "upgrade" type 3 to type 42 if the
number of glyphs overflowed. However, as `CharacterTracker` can suggest
a new subset for other reasons (i.e., multiple glyphs for the same
character or a glyph for multiple characters may go to a second subset),
we do need proper subset handling here as well.

Since that is now done, we can drop the "promotion" from type 3 to type
42, as we don't get too many glyphs in each embedded font.

Font features allow font designers to provide alternate glyphs or
shaping within a single font. These features may be accessed via special
tags corresponding to internal tables of glyphs.

The mplcairo backend supports font features via an elaborate re-use of
the font file path [1]. This commit adds the API to make this officially
supported in the main user API.

[1] https://github.com/matplotlib/mplcairo/blob/v0.6.1/README.rst#font-formats-and-features

Add font feature API to Text

The only thing that expected this to work is Type 3 fonts in the PDF
backend, but only to avoid an error when loading a range of characters
(not all of which would be used.) This would introduce an odd behaviour
in that `load_char` could never fail even if the glyph never existed.
You would instead end up with the `null` glyph from the last font.

Previously, in a mathtext string like `r"$\sin x$"`, a thin space would
(correctly) be added between "sin" and "x", but that space would be
missing in expressions like `r"$\max f$"`.  The difference arose because
of the slightly different handling of subscripts and superscripts
after the `\sin` and `\max` operators: `\sin^n` puts the superscript as
a normal exponent, but `\max_x` puts the subscript centered below the
operator name ("overunder symbol).  The previous code for inserting the
thin space did not handle the "overunder" case; fix that.  The new
behavior is tested by the change in test_operator_space, as well as by
mathtext1_dejavusans_06.

The change in mathtext_foo_29 arises because the extra thin space now
inserted after `\limsup` slightly shifts the centering of the whole
string.  Ideally that thin space should be suppressed if there's no
token after the operator, but that's not something currently implemented
either for e.g. `\sin` (compare e.g. the right-alignments in
`text(.5, .9, r"$\sin$", ha="right"); text(.5, .8, r"$\mathrm{sin}$", ha="right"); axvline(.5)`
where the extra thin space after `\sin` is visible), so this patch just
makes things more consistent.

Rename _in_subscript_or_superscript to the more descriptive
_needs_space_after_subsuper; simplify its setting in operatorname();
avoid the need to introduce an extra explicitly-typed spaced_nucleus
variable.

Remove forced fallback from FT2Font::load_char

Implement libraqm for vector outputs

This enables loading a non-initial font from collections (`.ttc` files).
Currently exposed for `FT2Font`, only.

This should allow listing the metadata from the whole collection, which
will also pick the right one if specified, though it will not load the
specific index yet.

For backwards-compatibility, the path+index is passed around in a
lightweight subclass of `str`.

Note, this only has an effect if set as the global font. Otherwise, just
the font name is recorded, and the TeX engine's normal lookup is
performed.

Add support for loading all fonts from collections