Add WebGL backend

[EricForgy]

From Slack:

Simon Danisch [6:30 PM]
@eric Forgy since I can't find the issue where you posted about webgl anymore, here is my comment about the webio backend:
The sort of backend you imagine seems to be very close to a plotly makie backend... I'd recommend just creating a plotly backend with Plotly.jl (or whatever package it was that has the better support for interactivity 😛 )

It would be a nice stop gap solution while we don't have a native webgl backend, which is quite a bit more involved
It should be pretty straight forward to get a first prototype running, since you just need to support a few basic plotting types 😉 The biggest difficulty for me so far was just figuring out all the different plotly attributes one needs to set, to e.g. make a "raw" line plot (no axis etc)
these are the basic attributes one needs to define, and you can also see what kind of parameters one needs to forward:
https://github.com/JuliaPlots/AbstractPlotting.jl/blob/master/src/interfaces.jl#L36
you can see the basic methods you need to overload for a backend here:
https://github.com/JuliaPlots/CairoMakie.jl/blob/master/src/CairoMakie.jl#L486
the draw_atomic overloads in CairoMakie may be quite a bit simpler with plotly, since you could just reuse the plotly camera + projection, for a first prototype 😉
so a first prototype should be possible in ~100locs