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
(cherry picked from commit 4bdf9a1e70e8aef0066fb3af72ad580069b02fa1)