avutil/display: use hypot().
avformat/mov: use av_display_rotation_get() for rotate metadata.
ffplay: support rotated video.

with -f lavfi -i testsrc=s=hd1080 as source:
  rotate=90*PI/180  vs transpose=clock:   42fps -> 64fps
  rotate=180*PI/180 vs vflip,hflip:       75fps -> 77fps
  rotate=270*PI/180 vs transpose=cclock:  43fps -> 63fps