Fix body pose bugs

[ziyuan-linn]

This PR fixes various bugs in the bodyPose model.

• Rename the confidence value for the entire pose from score to confidence.
• Fix the flipped and flipHorizontal not working issue by implementing mirroring on ml5's end.
• Fix the keypoints not resizing issue on MoveNet model.
  • It seems like MoveNet model's keypoints are scaled on the video's intrinsic size ("videoWidth" and "videoHeight") instead of display size ("width" and "height"). This contradicts the behavior of handPose, faceMesh, and the BlazePose model. This fix rescales the keypoints to the video display size.

[shiffman]

Looks great! This change might be worth doing a patch release to practice and develop a process for documenting releases? cc @sharellb @alanvww