ffplay: allow borderless playback windows

For a pure video tile effect, and enabling better integration of playback windows
into other programs. It would improve the looks in many situations and avoid ugly
hacks like this: http://stackoverflow.com/q/31465630/315024

Signed-off-by: Lucas Sandery <lucas-sandery@users.noreply.github.com>
Signed-off-by: Marton Balint <cus@passwd.hu>
