Fix #1893: active processing for panner/convolver/merger (#1941)

* Fix #1893: active processing for panner/convolver/merger

State that the number of chanels in the output is 1 if the node is not
actively processing any inputs.

* Add note about convolver diagrams

index.bs

@@ -6235,7 +6235,9 @@ This interface represents an {{AudioNode}} for
 combining channels from multiple audio streams into a single audio
 stream. It has a variable number of inputs (defaulting to 6), but not
 all of them need be connected. There is a single output whose audio
-stream has a number of channels equal to the number of inputs.
+stream has a number of channels equal to the number of inputs when any
+of the inputs is [=actively processing=].  If none of the inputs are 
+[=actively processing=], then output is a single channel of silence.
 
 To merge multiple inputs into one stream, each input gets downmixed
 into one channel (mono) based on the specified mixing rule. An
@@ -6756,6 +6758,11 @@ Developers desiring more complex and arbitrary matrixing can use a
 {{ConvolverNode}}s and a
 {{ChannelMergerNode}}.
 
+If this node is not [=actively processing=], the output is a single
+channel of silence.
+
+Note: The diagrams below show the outputs when [=actively processing=].
+
 <figure id="convolver-diagram">
 	<img alt="reverb matrixing" src="images/convolver-diagram.png">
 	<figcaption>
@@ -8355,8 +8362,10 @@ channels) and cannot be increased. Connections from nodes with fewer
 or more channels will be <a href="#channel-up-mixing-and-down-mixing">up-mixed or down-mixed
 appropriately</a>.
 
-The output of this node is hard-coded to stereo (2 channels) and
-cannot be configured.
+If the node is [=actively processing=], the output of this node is
+hard-coded to stereo (2 channels) and cannot be configured.  If the node
+is not [=actively processing=], then the output is a single channel of
+silence.
 
 The {{PanningModelType}} enum determines which
 spatialization algorithm will be used to position the audio in 3D