code: function( format, type, internal ){
      this.format   = format   || this.gl.RGB;
      this.internal = internal || this.format;
      this.type     = type     || this.gl.UNSIGNED_BYTE;
    }
docs: define underlying format, internal format and data type of texture
  @param {GLenum} [format  =GL_RGB] the pixel format, default to gl.RGB (can be gl.RGB, gl.RGBA, gl.LUMINANCE...)
  @param {GLenum} [type    =GL_UNSIGNED_BYTE] the pixel data type, default to gl.UNSIGNED_BYTE
  @param {GLenum} [internal=format] the pixel internal format, default to the same value than 'format' parameter (which must be in webgl 1)