drawElements(start, end) {
    const gl = this.gl_;
    this.getExtension('OES_element_index_uint');

    const elementType = gl.UNSIGNED_INT;
    const elementSize = 4;

    const numItems = end - start;
    const offsetInBytes = start * elementSize;
    gl.drawElements(gl.TRIANGLES, numItems, elementType, offsetInBytes);
  }
Execute a draw call based on the currently bound program, texture, buffers, attributes.
@param {number} start Start index.
@param {number} end End index.