[Cin] Rendering tests

[Andrea paz]

Taking advantage of the fact that I compiled a version that uses
system (external) ffmpeg, I did some comparative testing with CinGG
appimage which uses an internal ffmpeg instead.
I report the results:

Source: blender movie "Tears of steel" 1080p, webm
(Used an interval of the first 1440 frames via In/Out Points)

mp4; preset h264.mp4
ffmpeg internal --> render at 117.4 fps (25.4MB)
ffmpeg external --> render at 175.3 fps (22.8MB)

mxf; preset DNxHR 444
ffmpeg internal --> render at 92.9 fps (1.8GB)
ffmpeg external --> render at 120.5 fps (1.8GB)

I had already noticed that ffmpeg from the command line (external) was
faster than ffmpeg inside CinGG; but now we see that even external
ffmpeg called up inside CinGG is still faster than internal ffmpeg.
What could be the cause of this difference?