pixel perfect canvas rendering of box and block characters

[meganrogge]

Fixes #2409

[Tyriar]

Progress (canvas):

Old webgl rendering:

[Tyriar]

w3 alignment tests in canvas:

Looks like these are aligned wrong above: ▁▂▃▄▅▆▇█

old in webgl:

[Tyriar]

Webgl line height, letter spacing works. Plus the shade characters work 🎉

[Tyriar]

Had a think about testing and can't really think of a nice way to do it across all browsers unless we managed to get it drawing exactly across all browsers. But FF and Chrome seem to differ in how they draw corners of paths so not sure. Going to go ahead on this without tests for now, there is a button at the bottom of the demo that writes the alignment tests to the terminal though.

[Tyriar]

Moving ahead with this, here's canvas (1.1 line height, 1 letter spacing):

Webgl: