Geometry Nodes: enabled supports-mapping in nodes modifier

Sometimes the geometry nodes modifier does support mapping and
sometimes it does not. We have no infrastruture to determine this ahead
of time currently. In order to support common use cases, it makes sense
to add this flag to the modifier.

One such common use case is to use the mesh as surface that other
things are distributed on. Often, the distribution is controlled by vertex
groups. Therefore, it would be helpful if the modifier is evaluated
when the object is in vertex paint mode. This allows the user to see the
distributed objects while painting.

If the nodes modifier transforms the mesh in any way, vertex painting
might not work as expected anymore, because the `deformMatrices`
callback is not implemented. I'm not sure how this can be solved nicely, yet.

source/blender/modifiers/intern/MOD_nodes.cc

@@ -1079,9 +1079,9 @@ ModifierTypeInfo modifierType_Nodes = {
     /* srna */ &RNA_NodesModifier,
     /* type */ eModifierTypeType_Constructive,
     /* flags */
-    static_cast<ModifierTypeFlag>(eModifierTypeFlag_AcceptsMesh |
-                                  eModifierTypeFlag_SupportsEditmode |
-                                  eModifierTypeFlag_EnableInEditmode),
+    static_cast<ModifierTypeFlag>(
+        eModifierTypeFlag_AcceptsMesh | eModifierTypeFlag_SupportsEditmode |
+        eModifierTypeFlag_EnableInEditmode | eModifierTypeFlag_SupportsMapping),
     /* icon */ ICON_NODETREE,
 
     /* copyData */ copyData,