eventdemo: do not print pointer frames alone

eventdemo: do not print pointer frames alone

Print pointer frames only if any pointer related events are printed
first.

This avoids flooding the output with "pointer frame" just because of
motion. You can test this with e.g.
  $ ./weston-eventdemo --log-button

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Tested-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Benoit Gschwind <gschwind@gnu-log.net>
Tested-by: Benoit Gschwind <gschwind@gnu-log.net>