Add the MA_NODE_FLAG_SILENT_OUTPUT flag for nodes.

This flag is used to tell miniaudio that it should assume the output of
the node is silence. This is useful for special nodes that want to
output to a file and don't want the output of the node to contribute to
the final mix.

miniaudio.h

@@ -2184,6 +2184,15 @@ and include the following:
     |                                         | should set this for any nodes that perform        |
     |                                         | resampling.                                       |
     +-----------------------------------------+---------------------------------------------------+
+    | MA_NODE_FLAG_SILENT_OUTPUT              | Used to tell miniaudio that a node produces only  |
+    |                                         | silent output. This is useful for nodes where you |
+    |                                         | don't want the output to contribute to the final  |
+    |                                         | mix. An example might be if you want split your   |
+    |                                         | stream and have one branch be output to a file.   |
+    |                                         | When using this flag, you should avoid writing to |
+    |                                         | the output buffer of the node's processing        |
+    |                                         | callback because miniaudio will ignore it anyway. |
+    +-----------------------------------------+---------------------------------------------------+
 
 
 If you need to make a copy of an audio stream for effect processing you can use a splitter node
@@ -9965,7 +9974,8 @@ typedef enum
     MA_NODE_FLAG_PASSTHROUGH                = 0x00000001,
     MA_NODE_FLAG_CONTINUOUS_PROCESSING      = 0x00000002,
     MA_NODE_FLAG_ALLOW_NULL_INPUT           = 0x00000004,
-    MA_NODE_FLAG_DIFFERENT_PROCESSING_RATES = 0x00000008
+    MA_NODE_FLAG_DIFFERENT_PROCESSING_RATES = 0x00000008,
+    MA_NODE_FLAG_SILENT_OUTPUT              = 0x00000010
 } ma_node_flags;
 
 
@@ -67535,6 +67545,7 @@ static ma_result ma_node_input_bus_read_pcm_frames(ma_node* pInputNode, ma_node_
     ma_node_output_bus* pOutputBus;
     ma_node_output_bus* pFirst;
     ma_uint32 inputChannels;
+    ma_bool32 doesOutputBufferHaveContent = MA_FALSE;
 
     /*
     This will be called from the audio thread which means we can't be doing any locking. Basically,
@@ -67575,9 +67586,12 @@ static ma_result ma_node_input_bus_read_pcm_frames(ma_node* pInputNode, ma_node_
 
     for (pOutputBus = pFirst; pOutputBus != NULL; pOutputBus = ma_node_input_bus_next(pInputBus, pOutputBus)) {
         ma_uint32 framesProcessed = 0;
+        ma_bool32 isSilentOutput = MA_FALSE;
 
         MA_ASSERT(pOutputBus->pNode != NULL);
 
+        isSilentOutput = (((ma_node_base*)pOutputBus->pNode)->vtable->flags & MA_NODE_FLAG_SILENT_OUTPUT) != 0;
+
         if (pFramesOut != NULL) {
             /* Read. */
             float temp[MA_DATA_CONVERTER_STACK_BUFFER_SIZE / sizeof(float)];
@@ -67595,16 +67609,18 @@ static ma_result ma_node_input_bus_read_pcm_frames(ma_node* pInputNode, ma_node_
 
                 pRunningFramesOut = ma_offset_pcm_frames_ptr_f32(pFramesOut, framesProcessed, inputChannels);
 
-                if (pOutputBus == pFirst) {
+                if (doesOutputBufferHaveContent == MA_FALSE) {
                     /* Fast path. First attachment. We just read straight into the output buffer (no mixing required). */
                     result = ma_node_read_pcm_frames(pOutputBus->pNode, pOutputBus->outputBusIndex, pRunningFramesOut, framesToRead, &framesJustRead, globalTime + framesProcessed);
                 } else {
                     /* Slow path. Not the first attachment. Mixing required. */
                     result = ma_node_read_pcm_frames(pOutputBus->pNode, pOutputBus->outputBusIndex, temp, framesToRead, &framesJustRead, globalTime + framesProcessed);
                     if (result == MA_SUCCESS || result == MA_AT_END) {
+                        if (isSilentOutput == MA_FALSE) {   /* Don't mix if the node outputs silence. */
                             ma_mix_pcm_frames_f32(pRunningFramesOut, temp, framesJustRead, inputChannels, /*volume*/1);
                         }
                     }
+                }
 
                 framesProcessed += framesJustRead;
 
@@ -67623,12 +67639,21 @@ static ma_result ma_node_input_bus_read_pcm_frames(ma_node* pInputNode, ma_node_
             if (pOutputBus == pFirst && framesProcessed < frameCount) {
                 ma_silence_pcm_frames(ma_offset_pcm_frames_ptr(pFramesOut, framesProcessed, ma_format_f32, inputChannels), (frameCount - framesProcessed), ma_format_f32, inputChannels);
             }
+
+            if (isSilentOutput == MA_FALSE) {
+                doesOutputBufferHaveContent = MA_TRUE;
+            }
         } else {
             /* Seek. */
             ma_node_read_pcm_frames(pOutputBus->pNode, pOutputBus->outputBusIndex, NULL, frameCount, &framesProcessed, globalTime);
         }
     }
 
+    /* If we didn't output anything, output silence. */
+    if (doesOutputBufferHaveContent == MA_FALSE && pFramesOut != NULL) {
+        ma_silence_pcm_frames(pFramesOut, frameCount, ma_format_f32, inputChannels);
+    }
+
     /* In this path we always "process" the entire amount. */
     *pFramesRead = frameCount;