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/window-mixer.c

@@ -76,7 +76,7 @@ GtkWidget *create_mixer_controls(struct alsa_card *card) {
     }
 
     // create the gain control and attach to the grid
-    GtkWidget *w = make_gain_alsa_elem(elem, 1);
+    GtkWidget *w = make_gain_alsa_elem(elem, 1, WIDGET_GAIN_TAPER_LOG);
     gtk_grid_attach(GTK_GRID(mixer_top), w, input_num + 1, mix_num + 2, 1, 1);
 
     // look up the r_snk entry for the mixer input number