[Cgal plugin] Fix windows cmake dll path and add a scene example

[epernod]

• Not sure this is the good way to do it with cmake findCGal but otherwise the dll are not linked (if not on env_path)

• Add a quick scene test for cgal without image plugin.

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This PR:

• builds with SUCCESS for all platforms on the CI.
• does not generate new warnings.
• does not generate new unit test failures.
• does not generate new scene test failures.
• does not break API compatibility.
• is more than 1 week old (or has fast-merge label).

Reviewers will merge only if all these checks are true.

[guparan]

So I tried to play a bit with CGALPlugin and realized we can use CGAL in header-only mode. So, why should we copy some dlls?
I made a branch here: https://github.com/guparan/sofa/tree/clean_cgalplugin_cmake

EDIT: here is a patch to try my changes in this branch: cgal-plugin_fix.patch.txt

[guparan]

Did you have some time to test my patch @epernod ?

[epernod]

ok, it works if I don't build cgal, so in header only (only execute cgal-setup.exe, no visual build after this step)
Otherwise it fails at compilation: error: cannot open input file 'CGal.lib' when building plugin.

[guparan]

Header only usage should work even if CGAL has been built before. I will try to force header_only, there is a CMake variable for that in CGAL.

[epernod]

@guparan do you remember what is the status of CGal plugin config on windows?