Cannot convert multiple images

[ao6swind]

Hello, I found that when there are multiple tags in the HTML, the conversion to OpenXML fails.

For example:

<table>
  <tr>
    <td>
       <img src="https://domain/a.png">
       <img src="https://domain/b.png">
    </td>
  </tr>
</table>

This snippet will throw an error in ImagePrefetcher.cs.

I attempted to fix it,

[sonarqubecloud]

Quality Gate passed

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0 New issues
0 Accepted issues

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[onizet]

@ao6swind Very very good PR!
First it solves a nasty bug, but your code is very nice with a minimal locking time.

Just one small remark: GetIdOfPart is cross-thread safe as its code only look for the given part into its internal collection. The extra lock around it is unnecessary.
AddImagePart is already inside a lock statement so you make this right.