Visualization: RAM overflow for longer videos

[c-hoffmann]

I tried the visualization for different videos and noticed, that the shorter ones worked fine, the longer ones ran into problems. I was able to observe from the ffmpeg output that a "rawvideo" stream was created which grew very quickly. My system has 32GB RAM, which allows me to handle about 5 minutes of video (H.264, 960x1080 @ 29.97 fps). But it fills up my RAM almost completely. Anything beyond that leads to a RAM overflow, which can easily be observed with system monitor tools. On the console, the error message is "Broken Pipe" and "Conversion failed!". The code that seems responsible is connected to the read_video function.

Unfortunately, my python is a bit rusty and I was not able to hack a fix together quickly. I assume that the issue could be resolved by handling each frame of the videofile separately instead of loading them all into RAM as RGB24/rawvideo. Alternatively, loading the whole video into RAM as compressed images and converting single frames only for the output image might be feasible.

This might be connected to some of the issues in #108 , #152 and #204.
If necessary, I can provide console output, however this is easily reproducible since ffmpeg shows the rising output size and system monitor tools can be used to monitor RAM usage as well.

Fixing this would be highly appreciated, as we are using VP3D for running scientific research. Thank you for your work!