OpenSubdiv: Made it a modifier option to enable OSD for viewport

The idea of this commit is to make it so we can enable OpenSubdiv by default
for the release builds but keep it limited to the viewport only for a specific
meshes. This is a temporary solution for until all the needed features are
supported on the OpenSubdiv side.

Flag itself is done as a dedicated field in modifier DNA so we can easily
remove it in the future without ending up with some temporary flag hanging
around forever.

release/scripts/startup/bl_ui/properties_data_modifier.py

@@ -874,6 +874,8 @@ class DATA_PT_modifiers(ModifierButtonsPanel, Panel):
         col.label(text="Options:")
         col.prop(md, "use_subsurf_uv")
         col.prop(md, "show_only_control_edges")
+        if hasattr(md, "use_opensubdiv"):
+            col.prop(md, "use_opensubdiv")
 
     def SURFACE(self, layout, ob, md):
         layout.label(text="Settings are inside the Physics tab")