Allow custom near/far clip depth when using toWebGLEntities

[ianmackenzie]

When using toWebGLEntities to interop with other WebGL code, the automatic computation of near and far clip depth from scene bounds makes it very difficult to ensure the same near/far clip depths are used by both elm-3d-scene and custom WebGL code, meaning that depth sorting between elm-3d-scene and custom WebGL entities may end up being incorrect.

One possible fix would be to do the automatic computation of near/far clip depth in composite instead of in toWebGLEntities, and then pass those values in to toWebGLEntities; this would then mean that any code that called toWebGLEntities could pass its own custom near/far clip depths instead.