Seeking by clicking on the video window can be annoying, because
the user might click on it accidentally while eg. trying to get
focus on it, and ffplay seeks instead.

This commit changes that behaviour to seek only when the right
mouse button is used to click and drag on the window.

Signed-off-by: Vittorio Gambaletta <ffmpeg-dev@vittgam.net>
Signed-off-by: Marton Balint <cus@passwd.hu>