qmlgloverlay: try harder to not produce corrupt output on OpenGL context changes

Display the backup dummy texture when a buffer has not been set yet on the qsgtexture/custom qt item.

If the GL context is not shareable with the currently configured OpenGL context, ignore the input buffer.

assigned to @gstreamer-merge-bot

mentioned in commit ystreet/gst-plugins-good@3b4673eb

mentioned in commit ystreet/gst-plugins-good@c75bd325

added 6 commits

• fe2adcd6...13cba418 - 4 commits from branch gstreamer:master
• 3b4673eb - qml: also use the dummy texture when no buffer has been set
• c75bd325 - qml: don't use buffers that have invalid contents

Compare with previous version

I couldn't merge this branch: CI failed!

assigned to @gstreamer-merge-bot and unassigned @ystreet

merged

changed milestone to %1.19.1