pdf: Improve text with characters outside embedded font limits

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

Author: QuLogic