Fix ffmpeg_reader not selecting a default stream

[bzczb]

fix #869 #899
ffmpeg, at least the version that's installed automatically with moviepy, sometimes comes up with no default stream for a given input file. This PR does nothing more than set a default stream automatically when none is specified, which is the cause of the missing "video_fps" issue.

Unfortunately I fixed this bug a long time ago in a working branch, and can't find the file that raised this problem in the first place. I think it was a ".wmv" or ".asf".

[coveralls]

coverage: 81.835% (-0.07%) from 81.904%
when pulling 338335b on bzczb:i869-default-stream
into bc8d1a8 on Zulko:master.

[keikoro]

@bzczb Thanks for the submission – I don't have time to review its actual contents, but saw it didn't pass the formatting checks. Could you update the formatting?

[bzczb]

@keikoro formatting fixed. Can't get pre-commit setup in CONTRIBUTING.md to work any which way. 3.6 venv fails when doing pip install -e, 3.12 venv fails because it's not 3.6.

[OsaAjani]

I would need a few tests added to show the proper working of this, the line 645 by itself make me doubt this work perfectly as intended.

[bzczb]

I need to remake this PR to match latest master.

The line looks right to me, the intent is to select a default at the end instead of the middle, so the global_data variable has to be removed and new logic added.

[OsaAjani]

Oh, I missed the rewrite of return on parsing methods. I let you check what need to be updated for the new master and add some tests showing an example of this in work. Thanks a lot for your work !