Materialx

Updated the MaterialX library to the v1.37 specification. See the [v1.37 changelist](http://www.materialx.org/assets/MaterialX.v1.37REV2.Changelist.pdf) for full details.

Added
- Added a Shadow Map option to the viewer, supported by shadowing functionality in GLSL code generation.
- Added support for the 'uisoftmin', 'uisoftmax', and 'uistep' attributes, updating Autodesk Standard Surface to leverage these features.
- Added support for LookGroup elements.
- Added support for Clang 9.

Changed
- Updated the set of standard nodes to match the v1.37 specification.
- Unified the rules for NodeDef outputs, with all NodeDefs defining their output set through Output child elements rather than 'type' attributes.
- Replaced GeomAttr elements with GeomProp elements.
- Replaced backdrop nodes with Backdrop elements.
- Aligned Matrix33 and Matrix44 with the row-vector convention, for improved consistency with Imath, USD, and other libraries.
- Updated the stb_image library to version 2.23.

Added
- Added a Load Environment option to the viewer, allowing arbitrary latitude-longitude HDR environments to be loaded at run-time.
- Added an initial TextureBaker class, supporting baking of procedural content to textures.
- Added initial support for units, including the MaterialX\:\:Unit, MaterialX\:\:UnitDef, and MaterialX\:\:UnitTypeDef classes.
- Added support for unit conversion in shader code generation.
- Added support for Visual Studio 2019.

Changed
- Updated Autodesk Standard Surface to the latest interface and graph.
- Updated the PyBind11 library to version 2.4.3.

Added
- Added a Save Material option to the viewer.
- Added property accessors to PropertyAssign and PropertySetAssign
- Added Python bindings for TypeDesc and array-type Values.
- Added Python functions getTypeString, getValueString, and createValueFromStrings.
- Added support for GCC 9 and Clang 8.

Changed
- Updated the interface of readFromXmlFile and writeToXmlFile to support FilePath and FileSearchPath arguments.
- Extended Python bindings for FilePath and FileSearchPath.

Removed
- Deprecated Python functions typeToName, valueToString, and stringToValue.
- Removed deprecated Python functions objectToString and stringToObject.

Fixed
- Fixed the OSL implementation of roughness_dual.

Merged shader code generation and physically-based shading nodes from Autodesk's ShaderX extensions. Added a default MaterialX viewer based on GLSL shader generation.

Added
- Added the MaterialXGenShader library, supporting shader code generation in GLSL and OSL.
- Added the MaterialXRender library, providing helper functionality for rendering MaterialX content.
- Added the MaterialXView library, providing a default MaterialX viewer.
- Added the physically-based shading node library (libraries/pbrlib).
- Added a root-level 'cmake' folder, including a standard FindMaterialX module.
- Added a root-level 'resources' folder, including example materials and meshes.
- Added documents for the 1.37 specification.

Changed
- Moved the MaterialX data libraries from 'documents/Libraries' to 'libraries'.
- Updated MaterialX node definitions to the 1.37 specification.
- Updated the PyBind11 library to version 2.2.4.

Removed
- Removed customizations of PyBind11 to support Python 2.6. Only Python versions 2.7 and 3.x are now supported.

Added
- Added support for 'nodedef' attributes on MaterialX\:\:Node, integrating this usage into GraphElement\:\:addNodeInstance.
- Added the MaterialX\:\:GeomPropDef class for geometric input declarations.
- Added the Document\:\:getGeomAttrValue method.
- Added the ValueElement\:\:getResolvedValue method.
- Added support for the MATERIALX_SEARCH_PATH environment variable.
- Added support for GCC 8 and Clang 7.

Changed
- Added callbacks Observer\:\:onCopyContent and Observer\:\:onClearContent, and removed callback Observer::onInitialize.
- Moved the standard document library to the 'documents/Libraries/stdlib' folder.

Added
- Added support for interface tokens, including the MaterialX\:\:BindToken class and '[TOKEN]' syntax in filenames.
- Added support for Clang 6.

Changed
- Updated geometry token syntax from '%TOKEN' to '\<TOKEN\>'.
- Replaced readXIncludes boolean with a readXIncludeFunction callback in the XmlReadOptions structure.
- Combined individual options into an XmlWriteOptions argument for the XML write functions.
- Extended functionality of the vector and matrix classes.
- Updated the PyBind11 library to version 2.2.3.
- Updated the PugiXML library to version 1.9.

Fixed
- Fixed graph implementations of range, extract, tiledimage, and ramp4 nodes.