Screen Space Ambient Occlusion post effect

[mvaligursky]

Fixes: #1136
Based on #3092, thank you to @raytranuk for an excellent contribution. This PR in addition to the initial PR exposed few parameters and did few small changes related to parameters. I also make it to work on Webgl1.

The implementation is based on the code in Filament Engine: https://github.com/google/filament

example of using the effect on a camera:

camera.addComponent("script");
camera.script.create("ssao", {
    attributes: {
        radius: 5,
        samples: 16,
        brightness: 0
    }
});

[willeastcott]

Delete this line if not required?

[willeastcott]

Approving with various suggestions and comments.

[Maksims]

Can please the shader code be moved to a separate file? Making it more usable than it is when written within JS?
Editor users don't have the easy ability to copy it.

[mvaligursky]

I agree it's not ideal, but it matches the style of all post effects - it's a single file with all shaders inlined.
But definitely something to consider for the future, we have plans for the next generation of post effects (including node graph support and similar).