Feature/#484 fix combustion demo error

This should fix the error of the combustion demo failing with openGl errors due to another OpenGL context in the loading thread.

Works for me :)

By Martin Bellgardt on 2018-06-29T12:34:44 (imported from GitLab)

merged

By Sebastian Freitag on 2018-06-29T13:57:21 (imported from GitLab)

mentioned in commit 1dec08f0

By Sebastian Freitag on 2018-06-29T13:57:22 (imported from GitLab)