[read-fonts/cmap] Towards supporting format13 subtables

[behdad]

Doesn't work, but a start. I don't know why. I also didn't update the tests.

Fixes https://github.com/googlefonts/fontations/issues/1431

[dfrg]

This is looking pretty good for reading cmap13 (though I wonder if some of this code can be shared with 12). Skrifa has its own logic for selecting the appropriate subtable rather than just enumerating all records (https://github.com/googlefonts/fontations/blob/e64b757b887b020868c11d85d36e5cee2e30f86e/skrifa/src/charmap.rs#L335) and this will need to be integrated there as well.

[behdad]

I see. I'll wait for you to get it to it I think. :)

[dfrg]

Added a commit that unifies cmap 12 and 13 code and adds a couple tests for 13. Skrifa integration can be done in a followup.