Fix snapshot
fixes #2 by rebinding previously bound fbo when makeCurrent is called.
versions affected are all OpenFlipper versions that use the QOpenGLWidget (all versions built with Qt 5.4.4 or higher)
pls see https://www.graphics.rwth-aachen.de:9000/OpenFlipper-Free/OpenFlipper-Free/pipelines/5530 for ci status and test on windows and mac prior to merge.