code: function useShader(shader) {
           if (this.activeShader !== shader) {
             this.flush();
             this.activeShader = shader;
             this.activeShader.bind();
             this.activeShader.uniforms.uMatrix = this.uMatrix.val;
           }
         }
docs: Select the shader to use for compositing
  @name useShader
  @see me.GLShader
  @memberOf me.WebGLRenderer.Compositor
  @function
  @param {me.GLShader} shader a reference to a GLShader instance