Restore Optix5 version
Currently Optix5 is not supported on master, only with the dedicated branch: https://gitlab.inria.fr/pacanows/MRF/-/tree/malia_optix5_dev
This one is not up-to-date and is hardly maintained. It should either be reintegrated in the main branch using one the following:
- a #optix_version macro (set when running cmake, either automatically or manually) for conditionnal compilation of the relevant files.
- subidviding the optix backend repo into optix5 and optix65 folders. Eventually with a generic_optix folders containing all the common code.
- using c++ heritage to do the previous proposition.
This may allow managing multiple Optix installation (and maybe simpler to add optix7 compatibility later).