Lobe pruning shader optimization

Introduces the lobe pruning concept to MaterialX shaders generated by
either MayaUSD or LookdevX.

We look into shader graph implemnetation and find where we can cut the
graph to simplify it based on the value of the main lobe attributes.

This means a shader with a subsurface weight of zero will completely
prune the evaluation of the subsurface, while a value of 1 will
completely prune the evaluation of the dielectric base.

This provides interesting performance values:

```
MayaUSD: Tumbling speed of a stage with a 11x11x11 set of cubes with a red OpenPBR shader

                initial  optimized
Default light   434fps   441fps
Dome light       17fps   115fps

LookdevX/MaterialX: Tumbling speed of a heavy geometry scene from MAYA-136105 using OpenPBR surface

                initial  optimized
Default light   192fps   476fps
Dome light      7.6fps    65fps
```

Technical side: The code looks for nodes in the nodegraph implementation and checks any node that directly connects to the interface on a 0-1 slider:
- Mix nodes: If the mix value if 0 or 1, forward the value/connection of the bg or fg port to the node directly upstream. Delete node.
- Multiply nodes: If any value is zero, write zero to the corresponding port on the upstream node. Delete node.
- PBR nodes: If the weight parameter is zero, substitute with a no-op PBR node. Delete node.
This makes sure the ShaderGen works on a reduced codebase and greatly reduces code size and complexity by disconnecting unused subgraphs and skipping black PBR calls.

lib/mayaUsd/render/MaterialXGenOgsXml/CMakeLists.txt

@@ -8,8 +8,10 @@ target_sources(${PROJECT_NAME}
         OgsFragment.cpp
         OgsXmlGenerator.cpp
         ShaderGenUtil.cpp
+        LobePruner.cpp
         Nodes/SurfaceNodeMaya.cpp
         Nodes/TexcoordNodeMaya.cpp
+        Nodes/MayaDarkClosureNode.cpp
         PugiXML/pugixml.cpp
 )
 
@@ -55,6 +57,7 @@ set(HEADERS
     OgsFragment.h
     OgsXmlGenerator.h
     ShaderGenUtil.h
+    LobePruner.h
 )
 
 # -----------------------------------------------------------------------------

lib/mayaUsd/render/MaterialXGenOgsXml/GlslFragmentGenerator.cpp

@@ -5,6 +5,7 @@
 
 #include "GlslFragmentGenerator.h"
 
+#include "Nodes/MayaDarkClosureNode.h"
 #include "Nodes/MayaShaderGraph.h"
 #include "Nodes/SurfaceNodeMaya.h"
 #include "Nodes/TexcoordNodeMaya.h"
@@ -24,6 +25,7 @@
 
 #include <mayaUsd/render/MaterialXGenOgsXml/CombinedMaterialXVersion.h>
 #include <mayaUsd/render/MaterialXGenOgsXml/GlslOcioNodeImpl.h>
+#include <mayaUsd/render/MaterialXGenOgsXml/LobePruner.h>
 #include <mayaUsd/render/MaterialXGenOgsXml/OgsXmlGenerator.h>
 
 #include <MaterialXGenGlsl/GlslShaderGenerator.h>
@@ -208,6 +210,13 @@ GlslFragmentGenerator::GlslFragmentGenerator()
             "IM_texcoord_vector3_" + GlslShaderGenerator::TARGET, TexcoordNodeGlslMaya::create);
     }
 
+    registerImplementation(
+        MaterialXMaya::ShaderGenUtil::LobePruner::getDarkBaseImplementationName(),
+        MayaDarkClosureNode::create);
+    registerImplementation(
+        MaterialXMaya::ShaderGenUtil::LobePruner::getDarkLayerImplementationName(),
+        MayaDarkClosureNode::create);
+
     // The MaterialX transform node will crash if one of the "space" inputs is empty. This will be
     // fixed in 1.38.9. In the meantime we use patched nodes to replace those previously added in
     // the base class.