Blender Git Loki

Cycles: Fix wrong OpenGL context manipulation in some cases

When the BlenderSession is destructed from an initialization of a
temporary are the current active context is window, not the draw
manager.

Made it so context release/activation goes correct in this case.

Steps to reproduce an original issue:
- Compile in debug mode, so that asserts are active.
- Start Cycles viewport render.
- Hit shortcut to open file (Cmd-O / Ctrl-O).