Remove ttconv and implement Type-42 embedding using fontTools (matplotlib#20866)

* Type-42 fonts using fontTools

Split _backend_pdf_ps.get_glyphs_subset into two functions:
get_glyphs_subset returns the font object (which needs to be
closed after using) and font_as_file serializes this object
into a BytesIO file-like object.

* Remove ttconv

Move the test in lib/matplotlib/tests/test_ttconv.py to
lib/matplotlib/tests/test_backend_pdf.py. Actually it has not been
a test of ttconv for a while, since type-3 conversion has not been
using ttconv.

* Add several more droppable tables

Fonttools cannot subset these and drops them; avoid the warning.

* Don't crash if the loca table is missing

Also don't end up in an infinite loop if there is a larger gap
between breakpoints than we would like.

* Don't crash if glyph bounds cannot be found

* Log the name of unsupported fonts

FT2Font throws RuntimeError for some fonts

* Get names from the original font

Not the subsetted one, since in some cases fontTools.subset
does not produce a good name table.

* Document ttconv removal

* Apply fixes from code review

* Clean up some ttconv replacements

De-wrap a few lines now that we allow longer lines.

---------

Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Author: jkseppan

doc/api/next_api_changes/removals/20866-JKS.rst

@@ -0,0 +1,7 @@
+ttconv removed
+~~~~~~~~~~~~~~
+
+The ``matplotlib._ttconv`` extension has been removed. Most of its
+functionaliy was already replaced by other code, and the only thing left
+was embedding TTF fonts in PostScript in Type 42 format. This is now
+done in the PS backend using the FontTools library.

extern/meson.build

@@ -1,6 +1,5 @@
 # Bundled code.
 subdir('agg24-svn')
-subdir('ttconv')
 
 # External code.