Issues with qmlglsink

[xargs]

I've built Qt 5.12.4 from source and GStreamer from git on Ubuntu 18.04. 
I've run this simple test program:

https://github.com/GStreamer/gst-plugins-good/tree/master/tests/examples/qt/qmlsink

and it works fine.  But if I change the video source in main.cpp to anything
other than videotestsrc, I get a black window with no video.  For example,
if I change

GstElement *src = gst_element_factory_make ("videotestsrc", NULL);

to

GstElement *src = gst_element_factory_make ("v4l2src", NULL);

with no other changes, my camera's recording LED comes on, but the app
window is black.  gst-launch-1.0 v4l2src ! xvimagesink works from the
command line, as does gst-launch-1.0 v4l2src ! glupload ! glimagesink.

What am I doing wrong?



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