Fix an issue with PCLVisualizer producing empty screenshots

This (presumably) fixes #954. The fix was proposed in [this thread](http://www.pcl-users.org/pcl-visualization-PCLVisualizer-saveScreenshot-and-pcl-viewer-J-command-returning-black-image-td4028645.html).

I do not encounter the problem with my setup, adding this line does not change anything for me.