Handle buffer metadata in gstreamer elements
For example for screencasting the cursor is send by gnome-shell using spa_meta_cursor. Right now, it is not possible to receive this information using the pipewiresrc (nor to send such structs using pipewiresink).
As discussed on IRC, the best solution is probably a GstSPAMeta which contains the buffer. That way the normal API can be used to access the metadata.