Minor preparatory PRs for#29807:

1. Move listing of Type1 glyph widths to backend_pdf:
   DviFont.widths (the widths of each glyph in a font) is only used
   by backend_pdf to embed (classic, Type1) TeX fonts; deprecate that
   attribute and instead compute it where needed. This change is in
   preparation for supporting {xe,lua}tex, where DviFont can also refer to
   OpenType fonts which would be embedded via a different mechanism into
   the pdf file, and which can contain so many glyphs that explicitly
   listing the width of each one would be wasteful.

2. Rename the confusingly named dviread._mul2012:
   Dvi fixed-point values are in 12.20 format (12 bits before the decimal
   point, 20 bits after -- see e.g. the 20-bit bitshifts scattered in the
   dviread module), not 20.12. Rename the helper function accordingly.

3. Clarify doc for Tfm.design_size.:
   See spec intexdoc tftopl section 10 andtexdoc dvitype section 18.

PR summary

PR checklist

• "closes #0000" is in the body of the PR description tolink the related issue
• new and changed code istested
• Plotting related features are demonstrated in anexample
• New Features andAPI Changes are noted with adirective and release note
• Documentation complies withgeneral anddocstring guidelines