Materialx

Added
- Added support for shared library builds on Windows.
- Added support for 16-bit unsigned integer images in MaterialXRender.
- Added support for compound nodegraphs with user interfaces in shader and UI generation.
- Added headers for newly proposed MaterialX closures in OSL.
- Added a shader translation command to the viewer, assigned to the 'T' hotkey.

Changed
- Improved the memory efficiency of texture baking operations.
- Improved the compatibility of generated MDL code with Omniverse.
- Refactored image resolution logic into new methods ImageHandler\:\:getReferencedImages and MaterialX\:\:getMaxDimensions.
- Moved the viewer hotkey for GLSL code generation from 'S' to 'G' for consistency with other languages.

Fixed
- Fixed the Color3.asTuple and Color4.asTuple methods in Python

Updated the MaterialX library to the v1.38 specification. See the [v1.38 changelist](http://www.materialx.org/assets/MaterialX.v1.38.Changelist.pdf) for full details.

Added
- Added support for the generalized 'layer' node in Physically Based Shading.
- Added user controls for texture baking and wedge rendering in the [MaterialX Viewer](https://github.com/materialx/MaterialX/blob/main/documents/DeveloperGuide/Viewer.md).
- Added support for Nvidia's Material Definition Language (MDL) in MaterialX code generation.
- Added support for inline source code in Implementation elements.
- Added support for TargetDef elements.
- Added viewer rendering to cloud-based tests in GitHub Actions.
- Added support for Xcode 12.

Changed
- Updated the set of standard nodes to match the v1.38 specification, including significant improvements to the [Physically Based Shading](http://www.materialx.org/assets/MaterialX.v1.38.PBRSpec.pdf) nodes.
- Replaced specialized Material elements with material nodes, allowing more flexible material definitions and more consistent traversal.
- Unified the Input and Parameter element classes, simplifying the MaterialX API and client code.
- Updated the MaterialX viewer to use native classes for GLSL rendering and camera controls, opening the door to additional render frameworks in the future.
- Updated the prefiltered path for specular environment rendering in GLSL, providing a closer match with the Filtered Importance Sampling path.
- Updated the definition of Autodesk Standard Surface to version 1.0.1.
- Updated the definition of UsdPreviewSurface to version 2.3.
- Renamed the default branch from master to main.

Removed
- Removed support for the 'complex_ior', 'backfacing', 'viewdirection' and 'fresnel' nodes in Physically Based Shading.
- Removed support for the Color2 type.

Added
- Added software rendering tests for MaterialXRenderGlsl to GitHub Actions.

Changed
- Improved the robustness of context management in MaterialXRenderGlsl.

Fixed
- Added a missing VAO binding to GlslRenderer\:\:drawScreenSpaceQuad.

Added
- Added Render Transparency and Render Double-Sided options to the Advanced Settings panel of the viewer.
- Added viewer support for partially-transparent mesh partitions.
- Added a subsurface scattering approximation to MaterialX GLSL.
- Added a CMake option for building shared libraries on Linux and MacOS.
- Added support for the latest OpenImageIO interface.

Changed
- Improved the robustness of texture baking and shader translation.
- Unified the handling of missing images in generated GLSL.
- Moved CI builds from Travis and Appveyor to GitHub Actions.

Fixed
- Fixed a bug in code generation for custom BSDF/EDF graphs.
- Fixed rendering of single-channel textures in MaterialXRenderGlsl.

Added
- Added support for texture baking from Python, including new example script [baketextures.py](python/Scripts/baketextures.py).
- Added support for texture baking of materials with multiple UDIMs.
- Added support for floating-point render targets in ShaderRenderer and its subclasses, allowing for HDR texture baking.
- Added support for displacement shaders in generated OSL.
- Added the ShaderTranslator class, laying the groundwork for support of shader translation graphs.
- Added Python bindings for the Image class.

Fixed
- Fixed the alignment of environment backgrounds in the viewer.

Removed
- Removed the CopyOptions class, making it the default behavior of Document\:\:importLibrary to skip conflicting elements.

Added
- Added command-line options for mesh, light, and camera transforms to the viewer.
- Added command-line options for screen dimensions and background color to the viewer.
- Added a Light Rotation slider to the Advanced Settings panel of the viewer.
- Added utility methods Backdrop\:\:setContainsElements and Backdrop\:\:getContainsElements.
- Added backwards compatibility for OpenImageIO 1.x.
- Added support for GCC 10.

Changed
- Improved energy conservation and preservation computations in generated GLSL.
- Upgraded Smith masking-shadowing to height-correlated form in generated GLSL.
- Improved the robustness of tangent frame computations in MaterialXRender.
- Renamed Backdrop\:\:setContains and getContains to Backdrop\:\:setContainsString and getContainsString for consistency.

Fixed
- Fixed the GLSL implementation of Burley diffuse for punctual lights.
- Fixed the upgrade path for compare nodes in v1.36 documents.