Update Screenshot

The screenshot is automatically taken at the begin of the stage rendering cycles, if the user doesn't make a manual one. When he does manually take one, this one is preferred and no automatic screenshot will be taken from now on for that project.

Now works reliably on <4.0 Devices, the libgdx update must have fixed the issue.

(Successfull) Testrun: http://catrobatgw.ist.tugraz.at:8080/job/Catroid-custom-branch-test/545/
- One Test fails, but I really can't see how any of my changes impact that particular test, I can't reproduce it on my phone either. Please  correct me if I'm wrong.