exo: Improve motion event synchronization.

This improves the synchronization by deferring the client side
wait until commit time.

BUG=
TEST=wayland_motion_events

Review-Url: https://codereview.chromium.org/2563263002
Cr-Commit-Position: refs/heads/master@{#437783}
1 file changed