Fix polymorphic deallocation

[WeiPhil]

Hi,
Recently, I ran into an allocator mismatch crash during cleanup (specifically during Python garbage collection) when working with Mitsuba:

1. A dynamic subclass like PrincipledBsdf is compiled with alignas(16) for SIMD operations.
2. But its memory lifetime is handed over to Python as a raw, base Bsdf* pointer (so that Python takes over and deallocates it).

Since the current deallocator in nanobind does placement destructors + raw operator delete(void*) on base pointers, standard C++ dynamic delete routing seems to be bypassed during cleanup. On cleanup, this unfortunately crashes with a mismatched free in our internal build configuration where the default compiler alignment is 8.

I've worked on a fix that has been working for us internally as it reverts to standard type-safe deallocations when classes have virtual destructors.

To be honest, I'm not 100% sure this is the best fix for this niche problem, but it was the only way I could find to keep the allocators happy.

Let me know what you think or if you have a better alternative!
Best,
Philippe