Allow efficiently clearing depth and stencil buffers

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.