Fix crash in Palettize filter
The patch just wraps the d-poitner structure into std::experimental::propagate_const, which disables detaching of the QMap on every color-swatch access.
This propagate_const is still in 'experimental' land, the patch also includes a fallback inmplementation by Jonathan B. Coe, from GitHub: https://github.com/jbcoe/propagate_const
Edited by Dmitry Kazakov