message: Fix inline variable parsing
  This changelist fixes the parsing of inline source code variables, where a single bracket would incorrectly be interpreted as a prefix/suffix marker.
diff: MATERIALX_NAMESPACE_BEGIN
  +namespace
  +{
  +
  +const string INLINE_VARIABLE_PREFIX("{{");
  +const string INLINE_VARIABLE_SUFFIX("}}");
  +
  +} // anonymous namespace
  +
  ShaderNodeImplPtr SourceCodeNode::create()
  {
  return std::make_shared<SourceCodeNode>();
  @@ -97,17 +105,14 @@ void SourceCodeNode::emitFunctionCall(const ShaderNode& node, GenContext& contex
  {
  // An inline function call
  - static const string prefix("{{");
  - static const string postfix("}}");
  -
  size_t pos = 0;
  - size_t i = _functionSource.find_first_of(prefix);
  + size_t i = _functionSource.find(INLINE_VARIABLE_PREFIX);
  StringSet variableNames;
  StringVec code;
  while (i != string::npos)
  {
  code.push_back(_functionSource.substr(pos, i - pos));
  - size_t j = _functionSource.find_first_of(postfix, i + 2);
  + size_t j = _functionSource.find(INLINE_VARIABLE_SUFFIX, i + 2);
  if (j == string::npos)
  {
  throw ExceptionShaderGenError("Malformed inline expression in implementation for node " + node.getName());
  @@ -146,7 +151,7 @@ void SourceCodeNode::emitFunctionCall(const ShaderNode& node, GenContext& contex
  }
  pos = j + 2;
  - i = _functionSource.find_first_of(prefix, pos);
  + i = _functionSource.find(INLINE_VARIABLE_PREFIX, pos);
  }
  code.push_back(_functionSource.substr(pos));
  