Metal renderer

[migueldeicaza]

Currently we use CoreText to render glyphs which works great, but poses one problem: alternating attributes between characters can be quite costly (various test cases that change the attribute from glyph to glyph can show this performance problem). The solution is not trivial, as there is a conflict between per-cell coloring and composited glyphs.

Windows Terminal now has an MIT licensed atlas implementation that I could port:

microsoft/terminal#11623