Avoid waiting unnecessarily when the media player reaches exactly THUMBNAIL_POSITION. However we must now wait until libvlc updates the position before polling with libvlc_media_player_get_position().

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>
(cherry picked from commit 0c2844f9)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>