Forking off from #4060

Thinking about this and the fact that I want xterm.js rendering to be as great as possible, I think we should support multiple glyphs depending on the x-axis when necessary. For the dotted line specifically, say we had a dot of size window.devicePixelRatio with the same sized gap in-between, so there would be at most 2 possible variants for every glyph:

Even:
|* * * * |

Odd:
|* * * * *|
| * * * * |

To store this as a unique glyph we need to make the glyph key different, this could be done by asking what underline variant it is and then store it as a number, maybe capped at 4 (2 bits) or 8 variants (3 bits)?

This would allow us make a non-repeated curvy line and not restrict us to a single cell for both the up and down parts of the wave, if that ends up looking better of course.

Current:

Desired: