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.
GStreamer does not take pixel aspect ratio into account for overlay rectangles.
If we do so, we end up with subtitles that are displayed off-center.
Behave like any other GStreamer sink on this matter.
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.
