Instead of creating and destroying threads when starting and stopping renders, keep a single thread alive for the duration of the session. This makes it so all display driver OpenGL resource allocation and destruction can happen in the same thread. This was implemented as part of trying to solve another bug, but it did not help. Still I prefer this behavior, to eliminate potential future issues wit graphics drivers or with future Cycles display driver implementations. Differential Revision: https://developer.blender.org/D14086