audioconvert: add zeroramp and gap detection to audioconvert

Make a new zeroramp.duration and zeroramp.gap property on audioconver.
It detects N samples of silence before triggering a fade-in or fade-out
of the given zeroramp.duration.

The zeroramp.duration is by default 5ms and zeroramp.gap is set to 0.

When the zeroramp.gap is set to 0, the audioconver will not do any gap
detection but it will only do fade-out from the last sample when the IO
Buffer area is removed from a port.

This by default makes the audio adapter perform a fade-out when the last
input of the port mixer was removed and the mixer is no longer scheduled
and the IO Area removed from the audioconverter input port.

doc/dox/config/pipewire-client.conf.5.md

@@ -87,6 +87,7 @@ stream.properties = {
     #dither.noise = 0
     #dither.method = none # rectangular, triangular, triangular-hf, wannamaker3, shaped5
     #debug.wav-path = ""
+    #zeroramp.gap = 0
     #zeroramp.duration = 0.005
 }
 ```

doc/dox/config/pipewire-props.7.md

@@ -842,6 +842,23 @@ Dithering is only useful for conversion to a format with less than 24 bits and w
 disabled otherwise.
 \endparblock
 
+@PAR@ node-prop  zeroramp.gap = 0
+\parblock
+This instructs the audio converter to run gap detection. If zeroramp.gap consecutive
+silence samples are found, the audio converter will perform a fade-in or fade-out with
+new/old samples respectively.
+
+Fade-in and fade-out are important when the signal has a sudden changes to and from silence
+samples, which can cause loud pops and cracks.
+
+When an application uses the proper PipeWire pause and resume functions, fades will be
+performed automatically where needed. For applications that simply send silence when
+paused (chrome, ...), zeroramp.gap detection can be a workaround.
+
+This is disabled by default because it might corrupt the signal with excessive fades in
+case the signal has large silence gaps.
+\endparblock
+
 @PAR@ node-prop  zeroramp.duration = 0.005
 The duration of fade-in and fade-out of the signal in seconds on silence gaps.
 

doc/dox/config/pipewire-pulse.conf.5.md

@@ -100,6 +100,7 @@ stream.properties = {
     #dither.noise = 0
     #dither.method = none # rectangular, triangular, triangular-hf, wannamaker3, shaped5
     #debug.wav-path = ""
+    #zeroramp.gap = 0
     #zeroramp.duration = 0.005
 }
 ```