code: func (l *TableView) OnBeforeDraw(fn func(int, int, int, int)) {
  	l.mtx.Lock()
  	l.onBeforeDraw = fn
  	l.mtx.Unlock()
  }
docs: // OnBeforeDraw is called when TableView is going to draw its cells.
  // Can be used to precache the data, and make OnDrawCell faster.
  // Callback receives 4 arguments: first visible column, first visible row,
  // the number of visible columns, the number of visible rows