Allow to set line join and cap in CairoMakie

[devmotion]

It would be nice to be able to use a different line join style (Cairo.CAIRO_LINE_JOIN_BEVEL instead of the default Cairo.CAIRO_LINE_JOIN_MITER) with CairoMakie, and possibly also a different line cap.

This can be useful to avoid visually irritating and seemingly incorrect values: when I plotted a stochastic trajectory that is constrained to the domain [0, 1] I ended up with a plot that seemed to contain negative values. The same did not happen in a scatter plot, and after a while I realized that it was caused by the default line join. For my application I reimplemented https://github.com/JuliaPlots/CairoMakie.jl/blob/c54dd87ec687deed1e8bca2a0faf702cbb46be38/src/infrastructure.jl#L278-L286 and added Cairo.set_line_join(screen.context, Cairo.CAIRO_LINE_JOIN_BEVEL) before https://github.com/JuliaPlots/CairoMakie.jl/blob/c54dd87ec687deed1e8bca2a0faf702cbb46be38/src/infrastructure.jl#L283, but it would be nice if this could be set by a user more generally. Probably this should be done differently since I assume sometimes one might even want to use different settings for different line plots in the same figure.

[jkrumbiegel]

yeah this would be good, GLMakie doesn't have other line joins that's why it's not in AbstractPlotting and also not in CairoMakie. But I think we could add it to AbstractPlotting with the caveat that it will not have an effect on anything but CairoMakie, I guess. cc @SimonDanisch

[ffreyer]

Implemented by #3771 in Makie 0.21