Cubemap texture loading problem

(Tested with Cinematic3D initial commit https://codereview.qt-project.org/#/c/141651/)

Testcase:
1. Start Cinematic 3D
2. Press bottom-left settings icon
3. Switch ”3D Background”

Expected: Background cube map switches to sky.

Actual: Sometimes sky textures load OK, but sometimes not fully so background contains black or random content. To reproduce redo step 3. multiple times. Attached image showing this.

More information: Reproduced on 2 different macbooks and Nexus 6.