Research transcoding quality issues

[cyberj0g]

We have encountered visual artifacts in transcoded videos on multiple occasions. R&D efforts are required to look into possible causes. Some leads are:

• encoding with constant bitrate instead of constant quality
• not using correct parameters for Nvidia encoder
• "flush frames" used to clear Nvenc buffer between segments impacting encoder's internal state
• re-initialization between segments affecting the quality of software encoding
• suboptimal encoding parameters
• scaling interpolation specifics of scale_cuda filter used with Nvenc

The first step should be to create a list of transcoded videos, where the issues are present, along with source videos. One of the recent cases encountered (see videos side by side): #2654

CC: @Thulinma @thomshutt @AlexKordic