transparency and overdraw do not adjust render order in WGLMakie

[ffreyer]

Plots with transparency = true should be rendered after opaque plots, since they don't write to the depth buffer. If they don't they end up behind later drawn opaque plots even if they shouldn't be, because they don't communicate their depth value.

Plots with overdraw = true don't depth test at all (no read or write) so that they draw over every already drawn plot. They again don't have a way to communicate being at min depth, so if something is drawn later it can draw over an overdraw = true plot.

Both of these should have their own pass (or at least be sorted such that all the both-false plots render first.)

Example of it being a problem:

using WGLMakie
fig = Figure()
for (i, overdraw) in enumerate([false, true])
    for (j, transparency) in enumerate([false, true])
        Label(fig[i,j][1,1], "overdraw = $overdraw, transparency = $transparency", tellwidth=false)
        ax = LScene(fig[i, j][2,1])
        linesegments!(ax, Rect3f(Point3f(-1), Vec3f(2)), overdraw = overdraw, transparency = transparency)
        mesh!(ax, Sphere(Point3f(0), 1f0), color = :orange)
    end
end
fig