Scripting improvements

Based on user feedback, we’ve added a scripting Define symbol to benefit other plugins that want to support the Substance API.

Using a Define symbol, you can encapsulate all code specific to the Substance plugin inside #if #endif. This allows the source code, which requires access to the Substance API, to be compiled only if the Substance plugin is installed for the project.

We’ve also added ListInputs() to list input parameters in a Substance graph as well as a function to get the generated textures using GetGeneratedTextures().