Metaball: Evaluate metaball objects as mesh components

With the ultimate goal of simplifying drawing and evaluation,
this patch makes the following changes and removes code:
- Use `Mesh` instead of `DispList` for evaluated basis metaballs.
- Remove all `DispList` drawing code, which is now unused.
- Simplify code that converts evaluated metaballs to meshes.
- Store the evaluated mesh in the evaluated geometry set.

This has the following indirect benefits:
- Evaluated meshes from metaball objects can be used in geometry nodes.
- Renderers can ignore evaluated metaball objects completely
- Cycles rendering no longer has to convert to mesh from `DispList`.
- We get closer to removing `DispList` completely.
- Optimizations to mesh rendering will also apply to metaball objects.

The vertex normals on the evaluated mesh are technically invalid;
the regular calculation wouldn't reproduce them. Metaball objects
don't support modifiers though, so it shouldn't be a problem.
Eventually we can support per-vertex custom normals (T93551).

Differential Revision: https://developer.blender.org/D14593

source/blender/editors/object/object_add.cc

@@ -2771,25 +2771,6 @@ static const EnumPropertyItem convert_target_items[] = {
     {0, nullptr, 0, nullptr, nullptr},
 };
 
-static void object_data_convert_ensure_curve_cache(Depsgraph *depsgraph, Scene *scene, Object *ob)
-{
-  if (ob->runtime.curve_cache == nullptr) {
-    /* Force creation. This is normally not needed but on operator
-     * redo we might end up with an object which isn't evaluated yet.
-     * Also happens in case we are working on a copy of the object
-     * (all its caches have been nuked then).
-     */
-    if (ELEM(ob->type, OB_SURF, OB_CURVES_LEGACY, OB_FONT)) {
-      /* We need 'for render' ON here, to enable computing bevel #DispList if needed.
-       * Also makes sense anyway, we would not want e.g. to lose hidden parts etc. */
-      BKE_displist_make_curveTypes(depsgraph, scene, ob, true);
-    }
-    else if (ob->type == OB_MBALL) {
-      BKE_displist_make_mball(depsgraph, scene, ob);
-    }
-  }
-}
-
 static void object_data_convert_curve_to_mesh(Main *bmain, Depsgraph *depsgraph, Object *ob)
 {
   Object *object_eval = DEG_get_evaluated_object(depsgraph, ob);
@@ -2908,7 +2889,7 @@ static int object_convert_exec(bContext *C, wmOperator *op)
   const bool use_faces = RNA_boolean_get(op->ptr, "faces");
   const float offset = RNA_float_get(op->ptr, "offset");
 
-  int a, mballConverted = 0;
+  int mballConverted = 0;
   bool gpencilConverted = false;
   bool gpencilCurveConverted = false;
 
@@ -3332,22 +3313,14 @@ static int object_convert_exec(bContext *C, wmOperator *op)
         MetaBall *mb = static_cast<MetaBall *>(newob->data);
         id_us_min(&mb->id);
 
-        newob->data = BKE_mesh_add(bmain, "Mesh");
+        /* Find the evaluated mesh of the basis metaball object. */
+        Object *object_eval = DEG_get_evaluated_object(depsgraph, baseob);
+        Mesh *mesh = BKE_mesh_new_from_object_to_bmain(bmain, depsgraph, object_eval, true);
+
+        id_us_plus(&mesh->id);
+        newob->data = mesh;
         newob->type = OB_MESH;
 
-        Mesh *me = static_cast<Mesh *>(newob->data);
-        me->totcol = mb->totcol;
-        if (newob->totcol) {
-          me->mat = static_cast<Material **>(MEM_dupallocN(mb->mat));
-          for (a = 0; a < newob->totcol; a++) {
-            id_us_plus((ID *)me->mat[a]);
-          }
-        }
-
-        object_data_convert_ensure_curve_cache(depsgraph, scene, baseob);
-        BKE_mesh_from_metaball(&baseob->runtime.curve_cache->disp,
-                               static_cast<Mesh *>(newob->data));
-
         if (obact->type == OB_MBALL) {
           basact = basen;
         }