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.
There is no reason to be more strict than the API actually is, check for
a screencast session only when required, like for absolute motion events
and touch events.
Tested with https://gitlab.gnome.org/snippets/1122
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/1307
b8524504f4