How to record optical flow with more than one channels while using "PixelsAsFloat"= true

[KaiXin-Chen]

Hi I am trying to record some oprical flow data by manually driving a car around. However, I faced the following issues:

1. If I record the optical flows into the format of png, everything is compressed to integers and made the data inprecise.
2. if I use "PixelsAsFloat"= true in settings.json, then I start to record in the format in pfm. However, this pfm file only has one channel, storing optical flow in one channel does not make any sense to me.

How can I make the pfm files to have 2 channels? or what is the right way to record optical flow information while I am manually driving the car?

Note: same issue as #4463