Add support for premultiplied alpha flag detection when creating GdkMemoryTextures.
Also check if overlays have premultiplied alpha and if they do set this flag for
their video info, so it will be detected when creating a GdkMemoryTexture out of them.
Fixes performance issues with karaoke subtitles, as now GTK does not have to perform
unnecessary multiplication.
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.
