Simplify
Remove unused variable
Some cosmetic changes
Drop the pic_file XDV opcode

Instead just output a pdf:image special that DVIPDFMx already
understands. Less code to support XDV files overall.
Drop the set_glyph_string XDV opcode

It is really just an optimization of the XDV file size, which is not
that important IMHO given that we treat XDV as an intermediate format,
so just simplify the life for the XDV drivers.
Rename set_glyph_array → set_glyphs

Now that we no longer have a glyph string vs glyph array distinction.
Increase DVI id to 6

I’m going to introduce backward-incompatible changes in the next
commits.
Simplify native font definitions

We stopped using family and style names a while ago, so drop them now.
Use begin/end_reflect opcodes similar to TeX-XeT

Just for compatibility, and now that we no longer use opcode 251 for
pic_file.
Drop unused XDV flags
Drop AAT variation support

It worked only with the, now defunct, xdv2pdf driver. TODO make the
primitives issue a deprecation warning.