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.

* Examples: use .texture of input render targets.

* Assign null instead of undefined.

remove material.ambient from everything per discussion here: https://github.com/mrdoob/three.js/pull/5741#issuecomment-67893202

Also change use of setClearColorHex to setClearColor.