Allow for dynamic background color

[ianmackenzie]

The WebGL.clearColor option is only applied once (when the scene is initially rendered), meaning that it's not possible to have a dynamic background color using WebGL.clearColor alone. However, it should be possible for elm-3d-scene to work around this limitation by:

• always setting the clear color to be fully transparent
• rendering (as the very first entity) a full-screen quad using the actual specified background color, without touching the depth buffer
• and then rendering all other entities on top of that

[ianmackenzie]

Alternate solution: always set a transparent clear color in WebGL, and then have a wrapper <div> with a background color set.