code: function RenderBuffer( gl, format ){
    this._uid = _UID++;
    this.gl = gl;
    this.id = gl.createRenderbuffer();
  
    this.width  = 0;
    this.height = 0;
    this.format = format || gl.DEPTH_COMPONENT16;
  
    this._valid = false;
  
    // Dummy allocation needed
    // on some platform (chrome 57, osx, nvidia), gl.framebufferRenderbuffer raise 
    // an INVALID_OPERATION if a RB is attached before it storage is set.
    this._storage();
  
  }
docs: @class
  @classdesc RenderBuffer class manage renderbuffers
  
  @param {WebGLRenderingContext} gl webgl context the texture belongs to
  @param {GLenum} [format=DEPTH_COMPONENT16] the pixel internal format, default to gl.DEPTH_COMPONENT16