Implement more joins and caps

[mtsr]

Round join and cap (possible to render in a single draw call per line, instead of two), bevel joins, etc. See https://wwwtyro.net/2019/11/18/instanced-lines.html

[mtsr]

Round cap can possibly be implemented without any additional vertices using this suggestion by TimD on the GP discord:

btw, do you render extra cap meshes like in the article you linked? in my impl i just extend the billboard a bit in both directions and use the pixel shader to discard/alpha blend pixels to create a rounded end point

See also Gumhold, S. Splatting illuminated ellipsoidswith depth correction (full text not available)
https://www.researchgate.net/publication/220839256_Splatting_Illuminated_Ellipsoids_with_Depth_Correction/citations

or

https://www.researchgate.net/publication/220200701_High-Quality_Cartographic_Roads_on_High-Resolution_DEMs

[mtsr]

Miter joins were disabled when polyline was updated to 0.6. This will be done as needed.

[spacekookie]

Has there been any work on this? Does polyline currently support any caps? Because any 90 degree angles currently look pretty ugly :(

[brianchirls]

@spacekookie I have a PR at #71. You're welcome to use my branch until/unless it gets merged.

I would love any constructive feedback on the code.