guard MslProgram::updateAttributeList() to only run once per MslProgram build() invocation - instead of repeatedly

Author: ld-kerley

source/MaterialXRenderMsl/MslPipelineStateObject.h

@@ -294,6 +294,7 @@ class MX_RENDERMSL_API MslProgram
     InputMap _uniformList;
     std::unordered_map<std::string, std::string> _globalUniformNameList;
     // List of program input attributes
+    bool _attributeListComplete{false};
     InputMap _attributeList;
 
     std::unordered_map<std::string, ImagePtr> _explicitBoundImages;

source/MaterialXRenderMsl/MslPipelineStateObject.mm

@@ -298,6 +298,8 @@ int GetStrideOfMetalType(MTLDataType type)
         throw ExceptionRenderError(errorType, errors);
     }
 
+    _attributeListComplete = false;
+
     // If we encountered any errors while trying to create return list
     // of all errors. That is we collect all errors per stage plus any
     // errors during linking and throw one exception for them all so that
@@ -1515,6 +1517,11 @@ throw ExceptionRenderError(
         throw ExceptionRenderError(errorType, errors);
     }
 
+    if (_attributeListComplete)
+    {
+        return _attributeList;
+    }
+
     if (_shader)
     {
         const ShaderStage& vs = _shader->getStage(Stage::VERTEX);
@@ -1556,6 +1563,8 @@ throw ExceptionRenderError(
             }
         }
 
+        _attributeListComplete = true;
+
         // Throw an error if any type mismatches were found
         if (uniformTypeMismatchFound)
         {