Merge branch 'chest_graphics_fix' into 'master'

Fix chest graphics bug

Fixes the bug that occurs when golden chests spawn on tiles that already have `add_mos` additional graphics. Since the golden chest expects the chest graphic to be in a particular spot in the list, the act of replacing graphics with the "open chest" often creates unusual behavior like giant open chests.

**This MR is save safe.**

See merge request !799