With libraqm, string layout produces glyph indices, not character codes,and font features may even produce different glyphs for the samecharacter code (e.g., by picking a different Stylistic Set). Thus wecannot rely on character codes as unique items within a font, and mustmove toward glyph indices everywhere.

Currently, we split text into single byte chunks and multi-byte glyphs,then iterate through single byte chunks for output and multi-byte glyphsfor output.Instead, output the single byte chunks as we finish them, then do themulti-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 specificencoding like `cp1252`. Instead, switch to Latin-1, which correspondsexactly to the first 256 character codes in Unicode, and can be mappeddirectly with `ord`.

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

For character codes outside the embedded font limits (256 for type 3 and65536 for type 42), we output them as XObjects instead of using textcommands. But there is nothing in the PDF spec that requires anyspecific encoding like this.Since we now support subsetting all fonts before embedding, split eachfont into groups based on the maximum character code (e.g., 256-entrygroups for type 3), then switch text strings to a different font subsetand 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-Englishtext, this means it will become selectable and copyable again.Fixesmatplotlib#21797

For Type 3 fonts, add a `ToUnicode` mapping (which was added in PDF1.2), and for Type 42 fonts, correct the Unicode encoding, which shouldbe UTF-16BE, not UCS2.

These characters are outside the BMP and should test subset splittingfor 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`