Switch object magic validation to hash table lookup

[slouken]

Right now if you pass an invalid object to an SDL API, we check a magic pointer to verify that it's still valid, and we clear that pointer when we destroy the object. This is helpful, but not 100% foolproof if the object points to invalid memory. We can change the magic pointer validation to look up the pointer in a valid object hashtable so we don't crash in those cases.

We might also want an option to strip these checks out for performance and/or abort the program if they occur.

Fixes #9235