In 89a7e19, an API for converting "dvi glyph indices" (as stored
in a dvi file) to FreeType-compatible keys (either "indices into
the native charmap" or "glyph names") was introduced.  It was
intended that end users (i.e., backends) would check the type of
`text.glyph_name_or_index` ((A) int or (B) str) and load the glyph
accordingly ((A) `FT_Set_Charmap(native_cmap); FT_Load_Char(index);` or
(B) `FT_Load_Glyph(FT_Get_Name_Index(name));`); however, with the future
introduction of {xe,lua}tex support, this kind of type checking becomes
inconvenient, because {xe,lua}tex's "dvi glyph indices", which are
directly equal to FreeType glyph indices (i.e. they would be loaded with
`FT_Load_Glyph(index);`), would normally also be converted to ints.

This PR introduces a new API (`Text.index`) that performs this mapping
(via the new `DviFont._index_dvi_to_freetype`), always mapping to
FreeType glyph indices (i.e. one can always just call `FT_Load_Glyph`
on the result).  To do so, in case (A) it loads itself the native
charmap (something the end user needed to do by themselves previously)
and performs the cmap-to-index conversion (`FT_Get_Char_Index`)
previously implicit in `FT_Load_Char`; in case (B) it performs itself
the name-to-index conversion (`FT_Get_Name_Index`).  When {xe,lua}tex
support is introduced in the future, `_index_dvi_to_freetype` will
just return the index as is.

The old APIs are not deprecated yet, as other changes will occur for
{xe,lua}tex support (e.g. font_effects will go away and be replaced by
`.font.effects`, as {xe,lua}tex don't store that info in the pdftexmap
entry), and grouping all API changes together seems nicer (to only
require a single adaptation step by the API consumer).

Currently, this PR is mostly a proof of concept; only the svg backend
is supported (under rcParams["svg.fonttype"] = "none", the default).
However, there is a companion branch on the mplcairo repository, also
named "luadvi", which implements support for all output formats.

Example (requiring both this PR, and mplcairo installed from its luadvi
branch):
```
import matplotlib as mpl; mpl.use("module://mplcairo.qt")
from matplotlib import pyplot as plt

plt.rcParams["text.latex.engine"] = "lualatex"  # or "xelatex"
plt.rcParams["text.latex.preamble"] = (
    # {lua,xe}tex can use any font installed on the system, spec'd using its
    # "normal" name.  Try e.g. DejaVu Sans instead.
    r"\usepackage{fontspec}\setmainfont{TeX Gyre Pagella}")
plt.figtext(.5, .5, r"\textrm{gff\textwon}", usetex=True)
plt.show()
```
Font effects are supported by mplcairo, e.g.
`\fontspec{DejaVu Sans}[FakeSlant=0.2] abc`.

TODO:

- Fix many likely remaining bugs.
- Rework font selection in texmanager, which is currently very ad-hoc
  due to the limited number of fonts supported by latex.
- Implement rendering support in the (other) builtin backends.  In
  particular, the Agg (and, if we care, cairo) backend will require
  significant reworking because dvipng, currently used to rasterize dvi
  to png, doesn't support luatex-generated dvi; instead we will need to
  proceed as with the other backends, reading the glyphs one at a time
  from the dvi file and rasterizing them one at a time to the output
  buffer.  Working on the other backends is not very high on my priority
  list (as I already have mplcairo as playground...) so it would be nice
  if others showed some interest for it :-)