Add action-mode that only generates when the embedded image differs

[Johennes]

Is your feature request related to a problem? Please describe.

The action appears to have parameters to embed the source image in the generated PNG and SVG files. This information could be used to only regenerate the files if the embedded source differs from the current source file.

Describe the solution you'd like

Add a new action-mode à la embedded-diff that regenerates the files only if the embedded source differs from the source file (or when there is no embedded source at all).

Describe alternatives you've considered

The only current alternative is to use git refs which requires a non-shallow clone.

Additional context

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