This adds the option to limit the rate of change (slew rate) of an output that's mixed by a simple mixer.
To enable it, a positive number has to be added at the end (6th number) of the output scaler line of the mixer,
specifying the min rise time of this output.
E.g. O: 10000 10000 0 -10000 10000 20000 for a rise time of 2s, resp. a max slew rate of 0.5s^-1.
Signed-off-by: Silvan Fuhrer <silvan@auterion.com>