Improve line rendering

[ianmackenzie]

Native WebGL line rendering is basically terrible - can't reliably do anything other than 1 pixel wide lines and even that doesn't always seem to work. It would be better to try to implement a triangle-based solution of some sort. Possibly useful resources:

• https://mattdesl.svbtle.com/drawing-lines-is-hard
• https://wwwtyro.net/2019/11/18/instanced-lines.html