Currently, we split text into single byte chunks and multi-byte glyphs,
then iterate through single byte chunks for output and multi-byte glyphs
for output.

Instead, output the single byte chunks as we finish them, then do the
multi-byte glyphs at the end.

For a Type 3 font, its encoding is entirely defined by its `Encoding`
dictionary (which we create), so there's no reason to use a specific
encoding like `cp1252`. Instead, switch to Latin-1, which corresponds
exactly to the first 256 character codes in Unicode, and can be mapped
directly with `ord`.

By tracking both character codes and glyph indices, we can handle
producing multiple font subsets if needed by a file format.

For character codes outside the embedded font limits (256 for type 3 and
65536 for type 42), we output them as XObjects instead of using text
commands. But there is nothing in the PDF spec that requires any
specific encoding like this.

Since we now support subsetting all fonts before embedding, split each
font into groups based on the maximum character code (e.g., 256-entry
groups for type 3), then switch text strings to a different font subset
and re-map character codes to it when necessary.

This means all text is true text (albeit with some strange encoding),
and we no longer need any XObjects for glyphs. For users of non-English
text, this means it will become selectable and copyable again.

Fixes matplotlib#21797

For Type 3 fonts, add a `ToUnicode` mapping (which was added in PDF
1.2), and for Type 42 fonts, correct the Unicode encoding, which should
be UTF-16BE, not UCS2.

These characters are outside the BMP and should test subset splitting
for type 42 output in PDF.

- `test_backend_ps::test_type3_font` is covered by
  `test_backend_ps::test_multi_font_type3`
- `test_text::test_pdf_chars_beyond_bmp` is covered by
  `test_backend_pdf::test_multi_font_type3` and
  `test_backend_pdf::test_multi_font_type42`
- `test_text::test_pdf_kerning` is covered by
  `test_backend_pdf::test_kerning`
- `test_text::test_pdf_type42_kerning` is covered by
  `test_backend_pdf::test_kerning`