Export SVG files in the headless mode

[AlexisDrogoul]

To support advanced function in the gamaR Package. This would enable us to provide a super smooth video.

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Actually, I made a quick tour of what is available, and I think the solution can be:

to create a new kind of display which internally uses http://www.jfree.org/jfreesvg/ (a lightweight library)
to create the corresponding IGraphics object using the same library
to add an option to the headless experiments so that they use this display instead of the standard image Java2D one and retrieve SVG elements instead of BufferedImages (after that the library has everything necessary to save SVG files with the correct headers, etc. )
Not a lot of work, I guess, and most of it could be put in a separate plugin.

(moved from https://github.com/gama-platform/gama/issues/1778)