[examples] Validation: beam extension/compression

[alxbilger]

Add another common scene in the Validation folder: beam extension + compression.

To merge after #6216

sofa_video_r60_0010.mp4

[ci-build][with-all-tests]

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[fredroy]

I guess the frames where it is green for a short moment is the "rest" shape of the beam ?

[alxbilger]

@fredroy yes, and the flickering appears because the min/max values are not fixed. They are recomputed at each time step.