Make the code compatible with HLS

[keryell]

It looks like the current std::variant use causes some HLS breakage https://jira.xilinx.com/browse/CR-1093830

Not super clear which workaround can be used.
This probably requires to start from a minimal version of the path_tracer with only 1 std::variant and experiment with the workarounds.

• Try whether just adding https://en.cppreference.com/w/cpp/utility/variant/monostate in all the std::variant does not fix the issue by avoiding casting from uninitialized value at the first place.
• A first solution is to put each variant in a global buffer.
  Not even sure it will work if we do not have only 1 top level variable like https://github.com/triSYCL/path_tracer/blob/main/include/render.hpp#L33
  It will be probably super slow.
• Rewrite the code in a more functional way, like replacing hit(material_t&) -> bool by hit(material_t) -> material_t and just testing if the returned value is not the std::monostate