GTK expects GL textures to have premultiplied alpha. The ones we get
from GStreamer don't, leading to incorrect rendering of semitransparent
frames.
GTK 4.12 gained an API to set a different GL texture format, but it
won't help for older GTK versions. Plus, at the time of writing, it
causes a very slow download/upload path in GTK.
So, use a GTK GL shader node to premultiply the alpha without leaving
the GPU.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1312>
Create a single, global GDK GL context and the corresponding GStreamer
GL display and wrapped GStreamer GL context when initializing the first
sink and continue using that for all further sinks.
Additionally, don't create a full GStreamer GL context inside the sink
but only distribute the wrapped GL context in the pipeline so that
elements that actually need a full GL context can create one that is
sharing with that one. The sink itself does not need a full GStreamer GL
context.
Then inside the sink check that any GL memory that arrives was created
by a GL context that can share with the wrapped GDK GL context and only
then use it.
And lastly, use the correct GL contexts for a) creating a sync point and
b) actually waiting on it.
Fixes https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/318
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1099>
