Revert "Allow setting dirty when combc length is 0"

This reverts commit 4b3233a.

cell.go

@@ -58,7 +58,7 @@ func (cb *CellBuffer) SetContent(x int, y int,
 		// dirty as well as the base cell, to make sure we consider
 		// both cells as dirty together.  We only need to do this
 		// if we're changing content
-		if (c.width > 0) && (mainc != c.currMain || len(combc) != len(c.currComb) || !(len(combc) > 0 && reflect.DeepEqual(combc, c.currComb))) {
+		if (c.width > 0) && (mainc != c.currMain || len(combc) != len(c.currComb) || (len(combc) > 0 && !reflect.DeepEqual(combc, c.currComb))) {
 			for i := 0; i < c.width; i++ {
 				cb.SetDirty(x+i, y, true)
 			}