Allow efficiently clearing depth and stencil buffers

[ianmackenzie]

Right now to clear the stencil or depth buffer it is necessary to do a hack like drawing a full-screen quad. It would be useful (and presumably more efficient) to have something like some special Entity values WebGL.clearDepthBuffer and WebGL.clearStencilBuffer that would end up simply calling gl.clearDepth() or gl.clearStencil() when drawn.