(PhVideoEngine) Fix permanent seek if av_frame_seek returns a future time

Dear Martin,

There was a little issue in the modifications I sent for the video engine 2 weeks ago: sometimes, when seeking manually with the control panel or via LTC or MTC, the video engine could get stucked in a position where it would permanently try to seek to the right frame, at each refresh event. This PR fixes this by adding a little margin to the test done before seeking.

Best regards,

Timothée