code: function setBlendMode(mode, gl) {
           gl = gl || this.gl;
           gl.enable(gl.BLEND);
  
           switch (mode) {
             case "multiply":
               gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
               this.currentBlendMode = mode;
               break;
  
             default:
               gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
               this.currentBlendMode = "normal";
               break;
           }
         }
docs: set a blend mode for the given context
  @name setBlendMode
  @memberOf me.WebGLRenderer.prototype
  @function
  @param {String} [mode="normal"] blend mode : "normal", "multiply"
  @param {WebGLRenderingContext} [gl]