Add configurable taper to GtkDial

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.

src/gtkdial.h

@@ -65,6 +65,15 @@ int gtk_dial_get_round_digits(GtkDial *dial);
 void gtk_dial_set_zero_db(GtkDial *dial, double zero_db);
 double gtk_dial_get_zero_db(GtkDial *dial);
 
+// taper functions
+enum {
+  GTK_DIAL_TAPER_LINEAR,
+  GTK_DIAL_TAPER_LOG
+};
+
+void gtk_dial_set_taper(GtkDial *dial, int taper);
+int gtk_dial_get_taper(GtkDial *dial);
+
 /**
  * @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)