CI: add FFTW and enforce usage

[psychocoderHPC]

With #4868 FFTW was not added to the CI, therefore the shadography code is not compile tested.

The second commit fixes issues in the Shadowgraphy plugin that were not detected before because the CI was not checking this code due to the missing FFTW3 dependency.

• [x] rebase against #4894, currently the CI should fail because of the bug #4894 is fixing

[PrometheusPi]

@psychocoderHPC as predicted by you, the CI failed. Could you rebase?

[psychocoderHPC]

Add a second commit to fix issues in the Shadowgraphy plugin.

[psychocoderHPC]

@ComputationalRadiationPhysics/picongpu-maintainers ready to be merged