-Waggregate-return: used by some Qt clases extensively and not super
useful for this example. Supress it.
warning: "GL_GLEXT_VERSION" redefined: Perform the same workaround as
qmlglsink by defining the old gl/GL.h header guard if the new GL/gl.h
guard exists.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/752>
We need to pass the X11 display to GstGL or else it will
use its own X11 Display pointer, and the GL Context won't get shared
correctly on newer X servers
1. glcontextid function is replaced by gstreamer gst_gl_context_new_wrapped .
2. call gst_init before gst_gl_display_new , seems gst_gl_display_new depends
on gst_allocator_register , which only worked after gst_init called
3. flush gstreamer OpenGL context before using shared texture, fix
flicker problem.
https://bugzilla.gnome.org/show_bug.cgi?id=735566
