Add joints for line mark

[lsh]

Currently strokes have no cap at joints which becomes apparently on thicker strokes.

[KaNaDaAT]

In terms of performance, it would be ideal if this calculation could also be done in the shader. You would always need the line before and after. This can also be solved by making all lines accessible from the shader. See: https://github.com/KaNaDaAT/vega-webgpu/blob/main/src/shaders/line.wgsl