Cycles: Add an option to use ray marching for volume rendering

Null Scattering currently has performance and noise issues, and it will
take time to address them. For now add the previous Ray Marching back as
an option.

Co-authored-by: Brecht Van Lommel <brecht@blender.org>
Pull Request: https://projects.blender.org/blender/blender/pulls/146317

scripts/startup/bl_ui/properties_data_volume.py

@@ -128,6 +128,8 @@ class DATA_PT_volume_render(DataButtonsPanel, Panel):
         col.prop(render, "space")
 
         if scene.render.engine == 'CYCLES':
+            col.prop(render, "step_size")
+
             col = layout.column(align=True)
             col.prop(render, "clipping")