This completes some earlier work that did this change only in modularized code.

fixed a bug where the SSAOPass could not be used in conjuction with other passes which came afterward in the compositing order

It's much faster to just swap the buffers in AfterimagePass instead of doing a blit to copy them.

The last enabled EffectComposer pass is the only one that can really render to the screen, so we can set the renderToScreen flag in the passes automatically. The only case where EffectComposers don't render to the screen at all in the current examples is webgl_materials_skin, that sets up multiple EffectComposers and then combines their results. For that special case we now have a renderToScreen flag in EffectComposer.

This simplifies setting up EffectComposer and will make further enhancements a bit easier.

Now the same geometry and camera are shared between full-screen quad instances. A mesh is still created for each full-screen quad instance since that avoids repeatedly setting the material on the mesh in cases where a pass only needs one material.

The quad can be passed to renderer.render() directly without being a part of a scene.

This deduplicates a lot of code that used to be the same in a lot of postprocessing passes.