[WIP] Fix RID get_ptr_by_index access to freed objects

[lawnjelly]

RID_Alloc get_ptr_by_index was allowing access to objects that had previously been freed, causing crashes in the BVH.

This PR changes the function to iterate by allocation id rather than index, and provides protection against access to freed allocations.

Fixes #44973

Notes

• Thanks to extensive debugging by @pouleyKetchoupp the issue was tracked down to access to freed memory
• This is a rough PR for discussion as I need to check it with pouley and reduz, and reduz may want to make / decide on the final PR. In particular I'm unsure whether the validator_chunks index used should be p_index or idx, I'm just guessing by reverse engineering the existing code.
• There are probably multiple ways of fixing this.
• Let me know of preferences for different names for functions etc.

Update

This version is changed to iterate through the list by allocation ID rather than index .. it doesn't matter the order for the BVH, and I suspect the index is semi random anyway, and iterating by alloc_id is more efficient and simpler.

While we could just check for nullptr and access up to max_alloc sequentially, it is inefficient if there are a lot of invalid allocations in the list, because of the spinlock .. so I'm trying a single function to move an iterator on to the next valid alloc_id.

• We can alternatively iterate by index if desired, it isn't hugely different.
• The other alternative is to maintain a separate list of used elements, similar to the free list, but this may be overkill for the use case (although we should consider whether this would be generally useful functionality for RID_PtrOwners).
• Note that this implementation does a fair amount of naive silliness compared to having a used_list, like if there are 512 members in the scenario minimum chunk size, it will iterate through all 512, even if only 1 is active.

[lawnjelly]

Just to keep this updated, this bug is still active. This PR does fix it - we discussed it with @reduz at the time and if I remember correctly he expressed an interest in fixing it himself in a slightly different way. So may need some prodding to remember to fix this.

[lawnjelly]

Superseded by #57655 .