This approach has the major advantage that only parsing can fail (due to
a malformed specifier or memory allocation failure). Since parsing is
done generically, while matching is per-option, this will allow to
remove substantial amounts of error checking code in following commits.
The new code also explicitly allows stream specifiers to be followed by
additional characters, which should allow cleaner handling of optional
maps, i.e. -map <stream_specifier>?, which is currently implemented in a
hacky way that breaks when the stream specifier itself contains the '?'
character (this can happen when matching metadata). It will also allow
further extending the syntax, which will be useful in following commits.
This introduces some minor behaviour changes:
* Matching metadata tags now requires the ':' character in keys or
values to be escaped. Previously it could not be present in keys, and
would be used verbatim in values. The change is required in order to
know where the value terminates.
* Multiple stream types in a single specifier are now rejected - such a
specifier makes no sense.
* Non-existent stream group ID or index is now ignored with a warning
rather than causing a failure. This is consistent with program
handling and is required to make matching fail-free.
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