Fix frame recording

[Erol444]

Inside XoutFrames, when called __del__, self._video_recorder was always None, so it didn't properly close the .mp4 video file and it was invalid (video players failed). I guess GC did its thing beforehand.

    def __del__(self):
        if self._video_recorder:

TODO: Fix recording both right + stereo, currently right isn't recorded:

with OakCamera() as oak:
    left = oak.create_camera('left', resolution='800p')
    right = oak.create_camera('right', resolution='800p')
    stereo = oak.create_stereo(left=left, right=right)
    oak.visualize(stereo.out.disparity, record_path='stereo.mp4')
    oak.visualize(right, record_path='right.mp4')
    oak.start(blocking=True)