Respect prim writer ShouldPruneChildren when shading mode is useRegistry

Currently, there is no way to register a shader writer that handles all
connected plugs to the shader. For example, the writer might handle file
nodes connected to the shader, but useRegistry shading mode will invoke
FileTextureWriter unconditionally. It will write out a UsdUVTexture
shader, which useRegistry won't be able to connect to the shading network.

lib/mayaUsd/fileio/shading/shadingModeUseRegistry.cpp

@@ -247,6 +247,9 @@ class UseRegistryShadingModeExporter : public UsdMayaShadingModeExporter
             if (shaderPrim && !topLevelShader) {
                 topLevelShader = UsdShadeShader(shaderPrim);
             }
+            if (srcShaderInfo->ShouldPruneChildren()) {
+                iterDepGraph.prune();
+            }
 
             for (unsigned int i = 0u; i < dstPlugs.length(); ++i) {
                 const MPlug dstPlug = dstPlugs[i];