Some bindings can't handle the coexistence of
GtkMediaStream:prepared and gtk_media_stream_prepared.
Help them out by renaming the function to
gtk_media_stream_set_prepared, and rename
gtk_media_stream_unprepared as well, to match.
The existing entry points still exist, deprecated.
Update all internal uses.
Fixes: #4023
Remove a boatload of "or %NULL" from nullable parameters
and return values. gi-docgen generates suitable text from
the annotation that we don't need to duplicate.
This adds a few missing nullable annotations too.
This way, we can support external libraries providing implementations of
GtkMediaFile.
We also add a media backend called 'nomedia' that can be enabled to not
compile any support for GtkMediaFile. This is useful when people want to
statically compile GTK into an application that does not use media.
For now, this option is the default.
We also support a new environment variable GTK_MEDIA that allows
selecting the implementation to use.
GTK_MEDIA=help can be used to get info about the available
implementations.
