GStreamer fixes for frame by frame, looping, querying duration.

[ofTheo]

Through working on #6871 noticed that there are some existing issues with the ofGstVideoPlayer:

• frame by frame doesn't work when toggled on, during playback
• looping is sometimes problematic
• setSpeed and getDuration sometimes have issues querying the position.

Upgrades:

• there is new GStreamer flag GST_SEEK_FLAG_INSTANT_RATE_CHANGE for setting speed which allows you to setSpeed without specifying position.

[ofTheo]

closed by #7066