Create a GL context handler subclass of recently added context handler base class and
rework code to use it. This simplifies a lot of things, like: switching of importers
at runtime, handling context queries, sharing data between importers, etc.
There were various problems with importer loader code. One defect was that
it kept managing a single global list of available importers and marking
which one is currently used on it. This made it not work correctly for
multiple sink instances in single process and was not thread safe.
This commit changes importer loader code into a GstObject subclass, which
keeps its own list of importers per instance and unlike before makes it
possible to free this data from memory when destroyed.
Now only open modules are kept always in memory until process finishes
since we cannot unload them once loaded anyway.
Add new GStreamer plugin with custom video sink for Clapper video player.
The main difference is that unlike the old one, this does not operate using `GtkGLArea`
anymore, but processes and displays `GdkTextures` directly through `GtkPicture` widget.
Also this one is installed like any other GStreamer plugin, thus can be used via
`gst-launch-1.0` binary or even by other GTK4 apps if they wish to integrate it.
In order to not depend on GL stuff at build time, this plugin uses seperate GModules
called "importers" in order to import different kind of memories into GdkTexture. This
allows expanding its capabilities further then we were able to do before.
