Fix transparency for type 0 quads

Revert "kayak_core: Improve `Tree` methods"

Also added some additional helper methods and cleaned up some existing
ones

Re-implemented with new depth map (should be more performant)

kayak_font: Add `Grapheme` and `RowCol`

Fix `DownwardIterator`

Fixed issue where DownwardIterator would iterate through siblings once
finished iterating through descendants

Also added tests 🧪

Improved kayak_font

Avoids having to run the calculation every frame when extracting.

More consistent. Trims trailing spaces on lines.