function clearViewport() {
            if (viewport) {
                var dpr = viewport.devicePixelRatio;
                // use scissor to make sure only clear the viewport
                gl.enable(gl.SCISSOR_TEST);
                gl.scissor(viewport.x * dpr, viewport.y * dpr, viewport.width * dpr, viewport.height * dpr);
            }
            gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
            if (viewport) {
                gl.disable(gl.SCISSOR_TEST);
            }
        }
Update GBuffer
@param {clay.Renderer} renderer
@param {clay.Scene} scene
@param {clay.Camera} camera
@param {Object} opts