gstqtvideosink: fix memory leak and synchronization issue related to buffer reference counting

https://bugzilla.gnome.org/show_bug.cgi?id=736758
CMakeLists.txt: actually require GStreamer 1.2.0
QGst::Pad: strong ref() the event in sendEvent()
openglsurfacepainter.cpp: remove '+1' for right and bottom in QRECT_TO_GLMATRIX

The '+1' calculation is wrong now that QRectF is used instead of QRect.
Add setVideoSink() method to GraphicsVideoSurface