add an option to build the soundfont reproducibly

[mirabilos]

cf. https://wiki.debian.org/ReproducibleBuilds/OggSerialNumbers

std::numeric_limits<qint64>::max() is used as flag to keep the previous behaviour of srand(time(NULL)); and using rand() as the stream serial number, as it’s outside of the range of an int. Otherwise, the integer argument is used directly as the stream serial number.

I expect most people to use -S 0 when converting their soundfonts.

With this patch applied, with exact same options (using the new -S) and input, the output .sf3 file is byte-for-byte identical no matter when it is built.

[mirabilos]

I’ve chosen to use #20 for Debian for now, so I’d recommend to apply that one instead of this one.