lavfi/pan: allow negative gain parameters also for other inputs than the first named

Expands the parser to also accept the separator '-' in addition to
'+', and take the negative sign into consideration.

The optional sign for the first factor in the expression is already
covered by parsing for an integer.

Signed-off-by: Moritz Barsnick <barsnick@gmx.net>
Reviewed-by: Nicolas George <george@nsup.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>