EMSUSD-277 remove muted layers recusrsively.

Also, improve comments explaining why we add and remove muted layers.

lib/mayaUsd/commands/layerEditorCommand.cpp

@@ -618,9 +618,9 @@ class MuteLayer : public BaseCmd
             restoreSelection();
         }
 
-        // we perfer not holding to pointers needlessly, but we need to hold on to the layer if we
-        // mute it otherwise usd will let go of it and its modifications, and any dirty children
-        // will also be lost
+        // We prefer not holding to pointers needlessly, but we need to hold on
+        // to the muted layer. OpenUSD let go of muted layers, so anonymous
+        // layers and any dirty children would be lost if not explicitly held on.
         addMutedLayer(layer);
         return true;
     }
@@ -640,7 +640,7 @@ class MuteLayer : public BaseCmd
             stage->MuteLayer(layer->GetIdentifier());
         }
 
-        // we can release the pointer
+        // We can release the now unmuted layers.
         removeMutedLayer(layer);
         return true;
     }

lib/mayaUsd/utils/layerMuting.cpp

@@ -95,8 +95,25 @@ void addMutedLayer(const PXR_NS::SdfLayerRefPtr& layer)
 
 void removeMutedLayer(const PXR_NS::SdfLayerRefPtr& layer)
 {
+    if (!layer)
+        return;
+
     MutedLayers& layers = getMutedLayers();
     layers.erase(layer);
+
+    // Stop holding onto sub-layers as well, in case they were previously
+    // dirty or anonymous.
+    //
+    // Note: we don't check the dirty or anonymous status while removing
+    //       in case the status changed. Trying to remove a layer that
+    //       was not held has no consequences.
+    //
+    // Note: the GetSubLayerPaths function returns proxies, so we have to
+    //       hold the std::string by value, not reference.
+    for (const std::string subLayerPath : layer->GetSubLayerPaths()) {
+        auto subLayer = SdfLayer::FindRelativeToLayer(layer, subLayerPath);
+        removeMutedLayer(subLayer);
+    }
 }
 
 void forgetMutedLayers()