LowPassFilter: allow for filtering to be disabled

using bandwidth of 0 gives no filtering

src/lib/mathlib/math/filter/LowPassFilter2p.cpp

@@ -46,6 +46,10 @@ namespace math
 void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
 {
     _cutoff_freq = cutoff_freq;
+    if (_cutoff_freq <= 0.0f) {
+        // no filtering
+        return;
+    }
     float fr = sample_freq/_cutoff_freq;
     float ohm = tanf(M_PI_F/fr);
     float c = 1.0f+2.0f*cosf(M_PI_F/4.0f)*ohm + ohm*ohm;
@@ -58,6 +62,10 @@ void LowPassFilter2p::set_cutoff_frequency(float sample_freq, float cutoff_freq)
 
 float LowPassFilter2p::apply(float sample)
 {
+    if (_cutoff_freq <= 0.0f) {
+        // no filtering
+        return sample;
+    }
     // do the filtering
     float delay_element_0 = sample - _delay_element_1 * _a1 - _delay_element_2 * _a2;
     if (isnan(delay_element_0) || isinf(delay_element_0)) {