fix ssao rendering issue in LScene and update lighting docs

[ffreyer]

Docs update for JuliaPlots/GLMakie.jl#107.

Also fixes this issue with ssao in figures:

Currently SSAO distinguishes between scenes using a stencil buffer that is only written to if scene.clear = true. So I changed the default for LScene. This means an LScene will not render over other plots that are rendered before.

I tried a bunch of variations of

fig = Figure()
Menu(fig[1, 1])
s = LScene(fig[2, 1])
scatter!(s, rand(10))

including 3d plots with ssao, 2d without, 2d without fxaa and different orders of operations (Menu-scene-plot, scene-menu-plot, scene-plot-menu). The Menu is always on top when opened, which is what I thought might break.

This also means that backgroundcolor now works for LScene.

[ffreyer]

GLMakie is one version behind in the docs build...?

[SimonDanisch]

You should be able to bump the version in AbstractPlotting\docs\Project.toml

[SimonDanisch]

hm, or so I thought...