Type-1 subsetting

This reduces pdf file sizes when usetex is active, at the cost of
some complexity in the code. We implement a charstring bytecode
interpreter to keep track of subroutine calls in font programs.

Give dviread.DviFont a fake filename attribute and a get_fontmap
method for character tracking.

In backend_pdf.py, refactor _get_subsetted_psname so it calls a method
_get_subset_prefix, and reuse that to create tags for Type-1 fonts.
Mark the methods static since they don't use anything from the instance.

Recommend merging to main to give people time to test this, not to
a 3.10 point release.

Closes matplotlib#127.

Co-Authored-By: Elliott Sales de Andrade <quantum.analyst@gmail.com>

Author: jkseppan

doc/users/next_whats_new/type1_subset.rst

@@ -0,0 +1,9 @@
+PDF files created with usetex now embed subsets of Type 1 fonts
+---------------------------------------------------------------
+
+When using the PDF backend with the usetex feature,
+Matplotlib calls TeX to render the text and formulas in the figure.
+The fonts that get used are usually "Type 1" fonts.
+They used to be embedded in full
+but are now limited to the glyphs that are actually used in the figure.
+This reduces the size of the resulting PDF files.