backends: Screencast is not needed for remote desktop
The portal API requires a screencast session only for absolution motion with remote desktop, other methods including relative motion do not require a screencast session.
Besides, the screencast session is pretty restrictive as it limits to a single monitor.
There is no reason to be more strict than the API actually is, therefore relax the requirement for a screencast session only for absolute motion events.
Tested with https://gitlab.gnome.org/snippets/1122
Closes: #1307 (closed)