CVE-2020-36211: Data races in gfwx

In the affected versions of this crate, ImageChunkMut<’_, T> unconditionally implements Send and Sync, allowing to create data races.

This can result in a memory corruption or undefined behavior when non thread-safe types are moved and referenced across thread boundaries.

The flaw was corrected in commit e7fb2f5 by adding T: Send bound to the Send impl and adding T: Sync bound to the Sync impl.