Image export has been performed using platform dependent offscreen buffers
that are setup manually. Segfaults etc. were observed on several occasions.
This commit enables offscreen rendering through the GLCanvas using QT
functionality.
Pro:
- QT takes care of platform dependent things
- Other image formats than just png come for free
Con:
- The gfx module didn't depend on QT and image export could be scripted
  Scripting is still possible, but a GLCanvas must be created.
- A offscreen solution could be implemented but it would still depend on QT