Change stencil to scissor test in SSAO

[ffreyer]

This pr swaps out the stencil test for a scissor test, which works regardless of clear. This does have some other potential issues though. The ssao postprocessor should only run once per pixel. With the scissor test multiple overlapping scenes could be running it, making things darker than they should be. To avoid this somewhat I only allowed leaf scenes (i.e. no children) to run SSAO. This means that SSAO will be wrong if a scene with children has a different projection matrix than it's children, or if its children cover less area. (I think this should be very rare?)

See also: JuliaPlots/AbstractPlotting.jl#624
I'm pretty sure JuliaPlots/AbstractPlotting.jl#649 breaks SSAO again (it uses clear = false) which this pr should fix.

[ffreyer]

Yup it does break SSAO: (more or less the example from docs/lighting)

With this pr:

[ffreyer]

This will also be useful for JuliaPlots/AbstractPlotting.jl#677 because it allows us to use clear = false in LScene and Axis3, which in turn allows information to be visualized in the screen space scene of the parent figure.

[SimonDanisch]

Hm, I'm pretty sure i had pretty important reasons for using stencil tests, since they're much more complicated and the initial version used scissor tests... I need to try to remember the reason before we can merge this :D

[ffreyer]

This is just the ssao postprocessing, not general rendering (i.e. not any plot rendering). I set it up with the stencil buffer originally, see #97

[SimonDanisch]

Oooh, the code looked so similar to the window rendering code :D I see... Ok, lets merge this then!