Materialx

Updated the MaterialX library to the v1.35 specification. See the [v1.35 changelist](http://www.materialx.org/assets/MaterialX.v1.35.Changelist.pdf) for full details.

Added
- Added the MaterialX\:\:Visibility class.
- Added 'file', 'function', and 'language' attributes to MaterialX\:\:Implementation.
- Added 'node' and 'nodedef' attributes to MaterialX\:\:ShaderRef. In v1.35, these attributes define which NodeDef is referenced by a ShaderRef.
- Added a 'material' attribute to MaterialX\:\:MaterialAssign. In v1.35, this attribute defines which Material is referenced by a MaterialAssign.

Changed
- Removed the MaterialX\:\:LightAssign and MaterialX\:\:Light classes. In v1.35, this functionality is now handled by the MaterialX\:\:Visibility class.
- Removed the 'default' attribute from MaterialX\:\:ValueElement. In v1.35, this functionality is now handled by the 'value' attribute.
- Replaced the 'matrix' type with 'matrix33' and 'matrix44', and replaced the MaterialX\:\:Matrix16 class with MaterialX\:\:Matrix3x3 and MaterialX\:\:Matrix4x4.
- Renamed Material\:\:getMaterialAssigns to Material\:\:getReferencingMaterialAssigns.
- Changed the argument type for MaterialAssign\:\:setExclusive and MaterialAssign\:\:getExclusive to boolean.

Added
- Added support for graph-based implementations of nodes.
- Added support for subtree/subgraph pruning in traversals.
- Added NodeGraph\:\:topologicalSort and MaterialX\:\:printGraphDot methods.
- Added a File module to MaterialXFormat and MaterialXTest.

Changed
- Extended NodeGraph::flattenSubgraphs to support subgraph recursion.
- Added a searchPath argument to MaterialX\:\:readFromXmlFile.

Fixed
- Fixed an issue where connecting elements were not returned in graph traversal edges.

Added
- Added support for document validation, including the Document\:\:validate and Element\:\:validate methods.
- Added helper methods ValueElement\:\:getResolvedValueString and Element\:\:getNamePath.
- Added standard library document.