Add an optional deadband at the bottom of the GtkDial so that very
quiet signals can be squashed into the first 1% of the dial. This will
be used to make the level meters more realistic.
This commit adds support for piecewise linear interpolation tapers to
GtkDial and the gain widget so that the 4th Gen 4i4 volume knob taper
can be modelled correctly.
The taper was previously set to a log function which matched the
Clarett volume knob. Add linear option to match the Scarlett 4th Gen
input gain knobs and hardware level meters.
