code: func (cb *CellBuffer) Fill(r rune, style Style) {
  	for i := range cb.cells {
  		c := &cb.cells[i]
  		c.currMain = r
  		c.currComb = nil
  		c.currStyle = style
  		c.width = 1
  	}
  }
docs: // Fill fills the entire cell buffer array with the specified character
  // and style.  Normally choose ' ' to clear the screen.  This API doesn't
  // support combining characters, or characters with a width larger than one.