Render: support USD Hydra render delegates

Hydra is a rendering architecture part of USD, designed to abstract the
host application from the renderer. A renderer implementing a Hydra
render delegate can run in any host application supporting Hydra, which
now includes Blender.

For external renderers this means less code to be written, and improved
performance due to a using a C++ API instead of a Python API.

Add-ons need to subclass bpy.types.HydraRenderEngine. See the example in
the Python API docs for details.

An add-on for Hydra Storm will be included as well. This is USD's
rasterizing renderer, used in other applications like usdview. For users
it can provide a preview of USD file export, and for developers it
serves a reference.

There are still limitations and missing features, especially around
materials. The remaining to do items are tracked in #110765.

This feature was contributed by AMD.

Ref #110765

Co-authored-by: Georgiy Markelov <georgiy.m.markelov@gmail.com>
Co-authored-by: Vasyl-Pidhirskyi <vpidhirskyi@gmail.com>
Co-authored-by: Brian Savery <brian.savery@gmail.com>
Co-authored-by: Brecht Van Lommel <brecht@blender.org>

Pull Request: https://projects.blender.org/blender/blender/pulls/104712

doc/python_api/examples/bpy.types.HydraRenderEngine.py

@@ -0,0 +1,57 @@
+"""
+Base class for integrating USD Hydra based renderers.
+
+USD Hydra Based Renderer
+++++++++++++++++++++++++
+"""
+
+import bpy
+
+
+class CustomHydraRenderEngine(bpy.types.HydraRenderEngine):
+    # Identifier and name in the user interface.
+    bl_idname = "CUSTOM_HYDRA_RENDERER"
+    bl_label = "Custom Hydra Renderer"
+
+    # Name of the render plugin.
+    bl_delegate_id = "HdCustomRendererPlugin"
+
+    # Register path to plugin.
+    @classmethod
+    def register(cls):
+        super().register()
+
+        import pxr
+        pxr.Plug.Registry().RegisterPlugins(['/path/to/plugin'])
+
+    # Render settings that will be passed to the delegate.
+    def get_render_settings(self, engine_type):
+        return {
+            'myBoolean': True,
+            'myValue': 8,
+            'aovToken:Depth': "depth",
+        }
+
+    # RenderEngine methods for update, render and draw are implemented in
+    # HydraRenderEngine. Optionally extra work can be done before or after
+    # by implementing the methods like this.
+    def update(self, data, depsgraph):
+        super().update(data, depsgraph)
+        # Do extra work here
+
+    def update_render_passes(self, scene, render_layer):
+        if render_layer.use_pass_z:
+            self.register_pass(scene, render_layer, 'Depth', 1, 'Z', 'VALUE')
+
+
+# Registration
+def register():
+    bpy.utils.register_class(CustomHydraRenderEngine)
+
+
+def unregister():
+    bpy.utils.unregister_class(CustomHydraRenderEngine)
+
+
+if __name__ == "__main__":
+    register()