draw(renderer, viewport) {
		renderer.save();
		renderer.clipRect(0, 0, viewport.width, viewport.height);
		renderer.clearColor(this.color);
		renderer.restore();
	}
draw this color layer (automatically called by melonJS)
@param {CanvasRenderer|WebGLRenderer} renderer - a renderer instance
@param {Camera2d} [viewport] - the viewport to (re)draw