Change the autowah to have a cutoff range of 20Hz to 20KHz

This is the same as allowed in the SF2 spec for its filter cutoff generator,
which can be used for a wah effect in MIDI. It makes sense to use a similar
range here.

Alc/effects/autowah.c

@@ -108,7 +108,7 @@ static ALvoid ALautowahState_process(ALautowahState *state, ALuint SamplesToDo,
             gain = maxf(gain, GAIN_SILENCE_THRESHOLD);
 
             /* FIXME: What range does the filter cover? */
-            cutoff = lerp(1000.0f, (ALfloat)LOWPASSFREQREF, minf(gain/state->PeakGain, 1.0f));
+            cutoff = lerp(20.0f, 20000.0f, minf(gain/state->PeakGain, 1.0f));
 
             /* The code below is like calling ALfilterState_setParams with
              * ALfilterType_LowPass. However, instead of passing a bandwidth,