Add SVG Mime Type

[TheMDev]

When attempting to add SVG images to connectors and cables, they failed to show up even though the base64 content was correct in the HTML and SVG outputs.
Checking https://www.w3.org/TR/SVG11/intro.html reveals the mime type should be image/svg+xml, not image/svg.
As the code already handles mime subtype replacements for jpeg and tiff, adding svg here made for a simple fix.

[kvid]

Thank you for detecting and suggesting a fix for this bug. I tried your fix and I can verify that the HTML output looked correct, but I noticed that the SVG image for a connector is still not shown in the PNG output. I understand that such a problem is outside the scope of this PR, but I wonder if this is also observed by others. JPG, PNG, and GIF images in connectors seems to work fine, but a TIF image was rejected by Graphviz. It would be nice to create a test use case with all image types expected to work, and perhaps a separate issue for unexpected outputs for such a test use case.

The automatic checks are currently failing due to #441, that now should be fixed for PRs with dev as target, but I probably need to cherry-pick a commit into master to also fix PRs with master as target.

[kvid]

The commit mentioned above is now cherry-picked into master, but the automatc checks seem not to be re-executed automatically. Maybe we need to rebase this PR branch on top of the latest master (there should not be any conflicts) to re-execute the latest versions of these automatc checks?

@TheMDev - do you want to try doing this yourself, or should I do it?

[TheMDev]

I'm sorry for not getting back to you sooner. I cannot replicate your issue where an SVG image does not appear in a PNG output, but I did notice that the PNG output does not follow the same scaling as the HTML and SVG outputs. It seems this is because it uses a different render function that would likely require a rewrite. A more straightforward solution is to convert the SVG output to PNG at export. If you can provide more detailed information on the issue you noticed, I would be more than happy to look into it in this PR, but the scaling issue warrants a separate issue with more discussion on how to resolve it.

Ill rebase my branch onto master here shortly.