How to watch a 1080p video on retina display with a 1:1 pixel ratio (with no scaling up)?
You can't watch any full screen video that is not in the exact native display resolution without scaling. At least in my knowledge.

The question is - is the scaling good or not. You may try different players.

If you want pixel to pixel mapping, you can only set the player size to 100% of the video, and it'll show as one of the windows on the screen, depending on the video file resolution - but only that way will you have the pixel-accurate playback.

Somebody correct me if I'm wrong.