Add clearDepth attribute to RenderPass

[fordhurley]

This makes it possible to clear the depth buffer before a RenderPass, which is useful when trying to render overlays using a second scene. I was struggling with this limitation myself when I came across an unanswered Stack Overflow question with the exact same issue: http://stackoverflow.com/questions/40548144/multiple-scenes-rendering-using-three-renderpass

This solution works for our use case, and seems like it would be useful for at least one other person, so I figured I'd open a PR. It's very likely that there's some way to solve this using the WebGLRenderer options as is, but I haven't found it yet.

[mrdoob]

Thanks!