Add support for piecewise linear interpolation taper to GtkDial

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.

src/gtkdial.h

@@ -74,6 +74,13 @@ enum {
 void gtk_dial_set_taper(GtkDial *dial, int taper);
 int gtk_dial_get_taper(GtkDial *dial);
 
+void gtk_dial_set_taper_linear_breakpoints(
+  GtkDial      *dial,
+  const double *breakpoints,
+  const double *outputs,
+  int           count
+);
+
 /**
  * @brief Set the colors which this dial uses. String codes can be one of the following:
  *     A standard name (Taken from the X11 rgb.txt file)