AddAlternativeFormatImportPart produces malformed documents if both parent and child contain shape-lines or similar objects

**Description**

Document is malformed if I use AddAlternativeFormatImportPart on a document with a shape-line (or similar objects) in the root *and* the attached documents.

**Information**

- .NET Target: .NET Framework 4.6.2
- DocumentFormat.OpenXml Version: 2.12.3

**Repro**

Create 2 word documents "Template.docx" and "ExampleAttachment.docx". Insert a single shape-line in both.
Note this was originally discovered using a shape-line and an attached image, but it can work with a shape-line.

```csharp
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ReproAddAlternativeFormatImportPartCorruption
{
class Program
{
static void Main(string[] args)
{
var iterations = 1;
var outputPath = $"test_{DateTime.Now.ToString("yyyyMMdd-hhmmss")}.docx";
var attachmentPath = "ExampleAttachment.docx";
File.Copy("Template.docx", outputPath);

var attachmentBytes = File.ReadAllBytes(attachmentPath);
using (var doc = WordprocessingDocument.Open(outputPath, true))
{
var firstPar = doc.MainDocumentPart.Document.Body.FirstChild;

for (int i = 0; i < iterations; i++)
{
var attachmentHeading = $"{attachmentPath} {i}";
var xmlFileId = attachmentHeading
.Replace(".", "_")
.Replace(" ", "_");

var compositeElements = AddDocumentAttachment(attachmentBytes, attachmentHeading, xmlFileId, doc.MainDocumentPart, AlternativeFormatImportPartType.WordprocessingML);
foreach (var element in compositeElements)
{
firstPar.InsertBeforeSelf(element);
}
}
doc.Save();
}

}

private static IEnumerable AddDocumentAttachment(
byte[] fileData,
string attachmentHeading,
string xmlFileId,
MainDocumentPart mainPart,
AlternativeFormatImportPartType? alternativeFormatImportPartType)
{

// Document is automatically saved and closed onDispose.
var chunk = mainPart.AddAlternativeFormatImportPart(alternativeFormatImportPartType.Value, xmlFileId);
using (var chunkStream = chunk.GetStream(FileMode.Create, FileAccess.Write))
using (var w = new BinaryWriter(chunkStream))
{
w.Write(fileData);
w.Flush();
}

var altChunk = new AltChunk
{
Id = xmlFileId
};

mainPart.Document.Save();

return new OpenXmlCompositeElement[]{
altChunk
};

}
}
}
```

**Observed**

Open the file in Word (Office 2019, version 1808, build 10372.20060), get the following error:

> We're sorry. We can't open .docx because we found a problem with its contents [OK] [Details]

> Details:

> HRESULT 0x800004005

> Location: Part: /word/document.xml, Line: 0, Column: 0

**Expected**

Create a document that does not cause errors.

**Additional notes**

Initially I discovered this issue happening inconsistently - like, I would attach 4 documents and it would be okay, but 5 was too much. And the only attached objects within the document were images.

Now, I can raise it consistently with the method described above, using documents created from scratch. If needed, I can send the docx files that will reproduce this issue.

If I use the same process *without* a shapes-line drawn into one or both of the files, the error does not occur.