[Bug 754680] fix support for EGL through GstVideoGLTextureUploadMeta

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=754680

--- Comment #27 from Hyunjun Ko <zzoon at igalia.com> ---
Some notes that I found.

1) The case that uses decodebin (not playbin) in the pipeline.
- Couldn't get gl context earlier, because glsinkbin is not linked yet.
And then vaapi element gets gl context during preroll, which means that it
should change vaapidisplay to get it working on gl.

2) The case that uses playbin.
- playbin handles autoplug-query signal, so vaapi element could get gl context
before decoder start. And works fine.
( Note that the issue also happens on playbin3 because handling autoplug-query
signal is not implemented yet)

3) Why is glx working even if using decodebin.
Because vaapi glx display is compatible with vaapiX11display.

Then I tried victor's patch for display_change callback, but it's not working
for all media because it lost parse information during vaapidisplay change.
Some media is working, but not all as far as I tested.

IHMO, we should find another way rather than changing vaapi display
dynamically.
Maybe we can make X11 or Wayland display first(by env variable?) if we don't
know about gl thing , then when gl context is found, we can bind egl.

I'll try more :)

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