This closes #1448, speed up for checking merged cells (#1500)

Author: CHANTXU64

cell.go

@@ -1388,19 +1388,31 @@ func (f *File) prepareCellStyle(ws *xlsxWorksheet, col, row, style int) int {
 // given cell reference.
 func (f *File) mergeCellsParser(ws *xlsxWorksheet, cell string) (string, error) {
 	cell = strings.ToUpper(cell)
+	col, row, err := CellNameToCoordinates(cell)
+	if err != nil {
+		return cell, err
+	}
 	if ws.MergeCells != nil {
 		for i := 0; i < len(ws.MergeCells.Cells); i++ {
 			if ws.MergeCells.Cells[i] == nil {
 				ws.MergeCells.Cells = append(ws.MergeCells.Cells[:i], ws.MergeCells.Cells[i+1:]...)
 				i--
 				continue
 			}
-			ok, err := f.checkCellInRangeRef(cell, ws.MergeCells.Cells[i].Ref)
-			if err != nil {
-				return cell, err
+			if ref := ws.MergeCells.Cells[i].Ref; len(ws.MergeCells.Cells[i].rect) == 0 && ref != "" {
+				if strings.Count(ref, ":") != 1 {
+					ref += ":" + ref
+				}
+				rect, err := rangeRefToCoordinates(ref)
+				if err != nil {
+					return cell, err
+				}
+				_ = sortCoordinates(rect)
+				ws.MergeCells.Cells[i].rect = rect
 			}
-			if ok {
+			if cellInRange([]int{col, row}, ws.MergeCells.Cells[i].rect) {
 				cell = strings.Split(ws.MergeCells.Cells[i].Ref, ":")[0]
+				break
 			}
 		}
 	}