ffmetadata has inconsistent escaping

[Federico Carboni]

;FFMETADATA1
key=value\\
key2=value

The backslash here is used to escape itself, not the following newline character. Yet the ffmetadata demuxer will read a key with value 'value
\nkey2=value'.

This behavior can also be observed by creating an ffmeta file with ffmpeg and probing it with ffprobe.

# Double backslash for terminal escaping
$ ffmpeg -metadata key=value\\ any.ffmeta
...
Output #0, ffmetadata, to 'any.ffmeta':
  Metadata:
    key             : value\
    encoder         : Lavf58.45.100
$ ffprobe any.ffmeta
...
Input #0, ffmetadata, from 's.ffmeta':
  Metadata:
    key             : value\
                    : encoder=Lavf58.45.100
# The encoder metadata property is gone.
...

[Michael Lelli]

I've discovered if you use Windows-style newlines (\r\n) instead of Unix style (\n) you can avoid this bug.