Add RTF_USE_PNG for rtf to use PNG even DOT_IMAGE_FORMAT is not PNG

[zeerd]

Microsoft Word could not include SVG for RTF document.

So, I add a new TAG named RTF_USE_PNG(NO by default) to make doxygen generate PNG for rtf even DOT_IMAGE_FORMAT is not PNG.

For #10292

[albert-github]

The problem does not only occur with plantuml images but also with other imahes that are generated in svg format (e.g. call / caller graphs) and as far as I can tell they are not included here.
I think it would be better to see whether there is a way to include svg images natively into rtf documents.
Note that when looking with OpenOffice I do see the svg image (I just tested it with a call graph)

[zeerd]

I am using the Microsoft Word(2016) which could not include svg.
But, OpenOffice is a good idea that I missed.

I have no OpenOffice and tried libreOffice(6.4.7.2) which could not include svg, too.

And I find that Microsoft Word would zoom large png pictures out to suit the page, but libreOffice will not.

Seems there is not an easy solution for anything, sigh .

[zeerd]

I am not sure how to make call/caller graphs, but inherit_graph and coll_graph for c++ could swap between png and svg by the new tag now.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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