[server] Send frame event instead of flush on relative pointer motion
This solves for me not working relative pointer motion with grab/lock in
Looking at the Xwayland code it is clear, that it expects a frame event on
wl_pointer versions 5 and above after relative motion events. wl_pointer
version 5 support was added to KWayland in b6bd28ab.
If the cursor is locked no absolute motion events are sent. In this case to
make sure relative motion events are processed by the client, send the frame
event after every relative motion.
Test Plan: Manually with Neverball in Xwayland mode.
Subscribers: davidedmundson, kde-frameworks-devel
Maniphest Tasks: T8923
Differential Revision: https://phabricator.kde.org/D13257