mt_srand effectively performs a modulo % 2147483648 on positive integers over 32 bits, but with negative integers it instead adds 2147483648 to the value it gets.
Seeds with equal results:
2147483649 == 1
2147483648 == 0
2147483647 == -1
-2147483646 == 2
-2147483647 == 1
-2147483648 == 0
Importantly though, seeding with anything less than -2147483648 will always yield the same result as seeding with zero.