[viewer] Add Open3d backend about jiminy

[duburcqa]

It appears to be much slower than expected, at least to load first render meshes. Once fully loaded it is reasonably fast on both integrated and dedicated GPU though. Full CPU rendering is pretty laggy but this is expected.

[duburcqa]

For the record, the PR #826 is bringing many optimisation regarding panda3d rendering speed, for both offscreen and onscreen rendering. So it is even more likely today than replacing it by Open3d is not an option. As of today, all OS and GPU types are supported pretty well, expect for a few features for which workarounds exist. There is also an official port to web-assembly in the making. In the end, Panda3d is probably not going anywhere soon. Finally, adding one additional backend would be very time consuming for little advantage, since notebook rendering is not a serious use case and embedded video is a robust fallback at the time being. Closing.