qt/gloverlay: reset OpenGL state after Qt drawing
qt/gloverlay: reset OpenGL state after Qt drawing

Reset to the original OpenGL state as required by the GStreamer OpenGL
API contract.  Fixes output with a glimagesink element downstream.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/623>

pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good