Memory Leak, even when destroying windows

[Horki]

example:
I have run ex_4.rs sample with valgrind tool

==27342== LEAK SUMMARY:
==27342== definitely lost: 0 bytes in 0 blocks
==27342== indirectly lost: 0 bytes in 0 blocks
==27342== possibly lost: 0 bytes in 0 blocks
==27342== still reachable: 499,249 bytes in 238 blocks
==27342== suppressed: 0 bytes in 0 blocks
==27342==
==27342== For counts of detected and suppressed errors, rerun with: -v
==27342== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)