wayland: Fix missing relative motion events
Use isNull on QSizeF to check for a zero delta instead of comparing it with a default-constructed QSizeF, which in practice initializes to (-1.0,-1.0). This caused relative motion events to be omitted if the delta happened to be equal to (-1.0,-1.0), causing mouse jumping in some applications.
Signed-off-by: John Brooks john@fastquake.com (cherry picked from commit a1191bea)