Make smoothing of fres dependent on Q

src/rvoice/fluid_iir_filter.c

@@ -391,7 +391,12 @@ void fluid_iir_filter_calc(fluid_iir_filter_t *iir_filter,
     }
     else if(FLUID_FABS(fres_diff) > 0.01f)
     {
-        static const int fres_incr_count = FLUID_BUFSIZE;
+        int fres_incr_count = FLUID_BUFSIZE;
+        fluid_real_t num_buffers = iir_filter->last_q;
+        fluid_clip(num_buffers, 1, 8);
+        // For high values of Q, the phase gets really steep. To prevent clicks when quickly modulating fres in this case, we need to smooth out "slower".
+        // This is done by simply using Q times FLUID_BUFSIZE samples for the interpolation to complete, capped at 8.
+        fres_incr_count *= num_buffers;
         iir_filter->fres_incr = fres_diff / (fres_incr_count);
         iir_filter->fres_incr_count = fres_incr_count;
         iir_filter->target_fres = fres;