Add default stream codec for format

[kice]

Get default output codec from the output format.

[jlaine]

Hi! Would you mind fixing the linter error please?

[WyattBlue]

This PR lacks justification given its lines of code added. This line especially makes me doubt its quality. libx264 is an encoder, not a codec.

self.assertEqual(output.format.video_codec.name, 'libx264')

Due to this, and age / merge conflict issues, I'm going to close this PR.