qmlglsrc: Black frames with Mesa

Submitted by Julien Isorce @cap

Link to original bug (#796506)

Description

I installed Ubuntu 18.04 in VM (virtual box) and the example https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/tests/examples/qt/qmlsrc renders black frames.

I will attach logs that I can get while running the pipeline LIBGL_DEBUG=verbose MESA_DEBUG=1 MESA_XSYNC=1 SVGA_EXTRA_LOGGING=1 GST_GL_XINITTHREADS=1 ./grabqml