GitLab

Many filters accept user-provided data that is cumbersome to provide as
text strings - e.g. binary files or very long text. For that reason such
filters typically provide a option whose value is the path from which
the filter loads the actual data.

However, filters doing their own IO internally is a layering violation
that the callers may not expect, and is thus best avoided. With the
recently introduced graph segment parsing API, loading option values
from files can now be handled by the caller.

This commit makes use of the new API in ffmpeg CLI. Any option name in
the filtergraph syntax can now be prefixed with a slash '/'. This will
cause ffmpeg to interpret the value as the path to load the actual value
from.