code: func (cb *CellBuffer) SetDirty(x, y int, dirty bool) {
  	if x >= 0 && y >= 0 && x < cb.w && y < cb.h {
  		c := &cb.cells[(y*cb.w)+x]
  		if dirty {
  			c.lastMain = rune(0)
  		} else {
  			if c.currMain == rune(0) {
  				c.currMain = ' '
  			}
  			c.lastMain = c.currMain
  			c.lastComb = c.currComb
  			c.lastStyle = c.currStyle
  		}
  	}
  }
docs: // SetDirty is normally used to indicate that a cell has
  // been displayed (in which case dirty is false), or to manually
  // force a cell to be marked dirty.