Generate all pointer (mouse) events from the "frame" callback as this
is the intended behavior according to the wl_pointer_listener docs.
In practice it's unlikely users would notice any difference however
there are potentially subtle differences because events were previously
created before all the pointer data had been collected.
This also has some minor advantages as each frame event no longer
needs to detect if scrolling is needed and in the case of motion events:
calculate the time-stamp twice.
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