avcodec/nvenc: De-compensate aspect ratio compensation of DVD-like content.

For reasons we are not privy to, nvidia decided that the nvenc encoder
should apply aspect ratio compensation to 'DVD like' content, assuming that
the content is not bt.601 compliant, but needs to be bt.601 compliant. In
this context, that means that they make the following, questionable,
assumptions:

1) If the input dimensions are 720x480 or 720x576, assume the content has
an active area of 704x480 or 704x576.

2) Assume that whatever the input sample aspect ratio is, it does not account
for the difference between 'physical' and 'active' dimensions.

From, these assumptions, they then conclude that they can 'help', by adjusting
the sample aspect ratio by a factor of 45/44. And indeed, if you wanted to
display only the 704 wide active area with the same aspect ratio as the full
720 wide image - this would be the correct adjustment factor, but what if you
don't? And more importantly, what if you're used to ffmpeg not making this kind
of adjustment at encode time - because none of the other encoders do this!

And, what if you had already accounted for bt.601 and your input had the
correct attributes? Well, it's going to apply the compensation anyway!
So, if you take some content, and feed it through nvenc repeatedly, it
will keep scaling the aspect ratio every time, stretching your video out
more and more and more.

So, clearly, regardless of whether you want to apply bt.601 aspect ratio
adjustments or not, this is not the way to do it. With any other ffmpeg
encoder, you would do it as part of defining your input paramters or
do the adjustment at playback time, and there's no reason by nvenc
should be any different.

This change adds some logic to undo the compensation that nvenc would
otherwise do.

nvidia engineers have told us that they will work to make this
compensation mechanism optional in a future release of the nvenc
SDK. At that point, we can adapt accordingly.

Signed-off-by: Philip Langdale <philipl@overt.org>
Reviewed-by: Timo Rothenpieler <timo@rothenpieler.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

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