This allows to work around ANGLE antialiasing issue appearing when compositing WebGL framebuffer with transparent background color with HTML canvas element background (while keeping antialiasing on).

WebGLRenderer constructor now takes JSON object with few optional parameters:

    renderer = new THREE.WebGLRenderer { scene: scene,
                                         antialias: true,
                                         clearColor: 0x000000,
                                         clearAlpha: 0 };

Here is how to change clear color in runtime:

    renderer.setClearColor( 0xff0000, 1 );