Simplify how infinite input regions are handled

Some input related code in kwin is mislead by the fact that when the
input region is infinite, SurfaceInterface::input() is going to return
an empty QRegion object.

We cannot really do that because the client could have just set a valid
empty wl_region object to ignore all input events.

This change makes SurfaceInterface assign an actually infinite region
when a NULL input region has been passed to set_input_region().