This commit allows invoking user-defined Python 'hook' functions to extend
the USD export functionality.
Added support for registering subclasses of a new bpy.types.USDHook
type which may implement the hooks as member functions. Supported
hook functions are on_export() and on_material_export(). Also added
definitions and Python registration for USDSceneExportContext and
USDMaterialExportContext structs that encapsulate arguments
to these functions.
Pull Request: https://projects.blender.org/blender/blender/pulls/108823
Michael Kowalski
ecb3262bf0