For the cases where we read a fixed size from the selection (eg. imposing
limits for the clipboard manager), g_input_stream_read_bytes_async() might
not read up to this given size if the other side is spoonfeeding it content.
Cater for multiple read/write cycles here, until (maximum) transfer size is
reached.
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1198
MetaSelectionSource represents a primary/clipboard/dnd selection owner,
it is an abstract type so wayland/x11/etc implementations can be provided.
These 3 selections are managed by the MetaSelection object, the current
selection owners will be set there, and signals will be emitted so the
previous selection owner can clean itself up.
The actual data transfer is done through the meta_selection_transfer_async()
call, which will take a GOutputStream and create a corresponding
GInputStream from the MetaSelectionSource in order to splice them.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/320
