Variant context

[leroyvn]

Dear Mitsuba maintainers, I sometimes find myself in a situation where a temporary variant override could be useful. A context manager would be great in such cases:

mi.set_variant("scalar_rgb")

with mi.variant_context("llvm_rgb"):
    # Do some preprocessing faster with the LLVM backend
    ...

mi.render(my_scene)

Would you consider such a feature?

[njroussel]

I really like this and genuinely thought we already had this (I was surprised when my script failed). I'd welcome a PR for it!

[leroyvn]

I opened #700.