Need `Since` markers for various enum/flags/boxed `get_type()` functions
gst_video_overlay_format_flags_get_type() was added in 1.16, but the corresponding enum was added long long ago. We need to add the correct
Since markers so that bindings can correctly handle versioning.
The correct versions can be taken from the Debian packaging:
To solve this we should probably generate a static gtk-doc header file for all these, and then ideally have a test that checks that this header file stays up to date (I'll complain when updating the Rust bindings, but something automatic would be better).
This also affects GStreamer core.