Refactor RasterFont and Glyphs to be simpler

[pushfoo]

TL;DR: Separating glyphs and metadata into two tables may have over-complicated the design

When first written, @dataclass didn't play nice with Pillow image cores. Since this type is painful to use / annotate, it may be best to take another approach.

One possible solution is to redesign the class as follows:

class RasterFont(MutableMapping[str, Glyph]):
   # Rember to add getmask2, which returns a tuple + is expected by PIL's font renderer now?
  ...