Can't reopen and save an Excel File, Exception with message : There are multiple root elements. Line 25, position 19.'

**Do you want to request a *feature* or report a *bug*?**
- [x] Bug

**Version of ClosedXML**

e.g. 0.92.0

## Reproducibility
When running the following code, we have an Exception into the method GenerateVmlDrawingPartContent. The file xl/drawings/vmlDrawing9.vml, has 2 root elements.

**Code to reproduce problem:**
```c#
public void Main()
{
string target = @"D:\temp\Export_closedXml.xlsx";
using (var wb = new XLWorkbook(@"D:\temp\closedXml\Export1.xlsx"))
{
wb.SaveAs(target);
}

using (var wb = new XLWorkbook(target))
{
wb.SaveAs(target);
}
}
```
- [x] I attached a sample spreadsheet. (You can drag files on to this issue)
[Export1.xlsx](https://github.com/ClosedXML/ClosedXML/files/2111969/Export1.xlsx)

Start with GenerateVmlDrawingPartContent, using the attached Export1.xlsx and the provided Main method to reproduce the failure during the second save. Inspect how xl/drawings/vmlDrawing9.vml is read and written; done means the workbook can be opened, saved, reopened, and saved again without the multiple-root-elements exception.

Written by the indexing model from the issue text.