Previously we don't merge material cached list, it create dangling pointer and memory leak.
Now we merge material cache list during the scene merge, and remove material in this list during the library free.
Reviewers: agoose77, dfelinto, hg1, pgi, campbellbarton, moguri
Reviewed By: campbellbarton, moguri
Subscribers: campbellbarton, youle, kupoman
Projects: #game_engine
Differential Revision: https://developer.blender.org/D1278