Fix failing Nvidia unit tests related to pts < dts errors when building with ffmpeg v4.4

[jailuthra]

Quite a few NVIDIA unit tests fail when compiling with ffmpeg v4.4 in #246

The number of pts < dts warning logs also significantly increased with v4.4, might be related to #227

[jailuthra]

We've started seeing these timestamp ordering errors due to:

1. An upstream change in how DTS (decode timestamp) is calculated by nvenc FFmpeg/FFmpeg@9ce7de9
2. We didn't use B-frames with NVIDIA until recently ffmpeg: Enable B-frames with GPU encoding #243 - where DTS calculation actually matters.
3. Default upstream value for B-frames was also 0 before FFmpeg/FFmpeg@a0949d0

Enabling B-frames with the new DTS calculation logic, still works for the most part - except when we have output fps: 0 i.e timestamps pass-through the transcoder verbatim.

As a temporarily solution in eae3d07 I've force-disabled B-frames when transcoding on GPU with fps: 0.