Bits per coded sample

[JoeSchiff]

Tests for #1203.

It seems there are 2 properties in ffmpeg called bits_per_coded_sample:

AVCodecParameters::bits_per_coded_sample
https://ffmpeg.org/doxygen/6.0/structAVCodecParameters.html#a3866500f51fabfa90faeae894c6e955c

AVCodecContext::bits_per_coded_sample
https://ffmpeg.org/doxygen/6.0/structAVCodecContext.html#a5273a8cf73ef598d632c6eee1379d5ad

I assume we want the one from AVCodecContext, so I changed the docstring to match.

I wrote 3 tests for the QT RLE decoder:
one for a valid value,
one for a invalid value,
and one to catch the encoder setter warning.

Here are the valid values:
https://ffmpeg.org/doxygen/6.0/qtrle_8c.html

[WyattBlue]

I decided to throw an error instead of merely deprecating setting for encoders. I put my changes on the original PR #1203 and make changes to the docstring #1331 to fix the doc test.

[JoeSchiff]

I'm confused as to what happened here. I'm concerned about not getting proper attribution as I don't see my name on my code here.

[WyattBlue]

You are credited in 7a5c2d1