This adds in-terminal rendering for the powerline glyphs E0B4 and E0B6,
similar to how we are rendering the triangle shapes currently.
The circle glyphs use a much more complex rendering due to the nuances
of drawing them: we use a midpoint algorithm for drawing on a 4x
supersampled matrix, fill, and then downsample. We use the same
downsampling approach as is done in the arc box drawing code.
The midpoint variant we're using here is described by Dennis Yurichev:
https://yurichev.com/news/20220322_circle/, although there are similar
variants elsewhere (some cited at the bottom of his article).
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