Modifiers: Simple Deform & Build, DerivedMesh → Mesh

This commit introduces `EditMeshData`. The fields in this struct are
extracted from `EditDerivedBMesh` into their own struct `EditMeshData`,
which can then also be used by the `Mesh` struct. This allows passing
deformed vertices efficiently to the draw routines.

The modifier code constructs a new Mesh instead of writing to ob->data;
even when ob->data is a CoW copy, it can still be used by different
objects and thus shouldn't be modified by a modifier.

source/blender/modifiers/intern/MOD_util.c

@@ -34,9 +34,11 @@
 
 #include "DNA_image_types.h"
 #include "DNA_meshdata_types.h"
+#include "DNA_mesh_types.h"
 #include "DNA_modifier_types.h"
 #include "DNA_object_types.h"
 #include "DNA_scene_types.h"
+#include "DNA_scene_types.h"
 
 #include "BLI_utildefines.h"
 #include "BLI_math_vector.h"
@@ -229,6 +231,20 @@ void modifier_get_vgroup(Object *ob, DerivedMesh *dm, const char *name, MDeformV
 	}
 }
 
+/* TODO(sybren): replace the above function with this one, once we got rid of DerivedMesh for modifiers. */
+void modifier_get_vgroup_mesh(Object *ob, struct Mesh *mesh, const char *name, MDeformVert **dvert, int *defgrp_index)
+{
+	*defgrp_index = defgroup_name_index(ob, name);
+	*dvert = NULL;
+
+	if (*defgrp_index != -1) {
+		if (ob->type == OB_LATTICE)
+			*dvert = BKE_lattice_deform_verts_get(ob);
+		else if (mesh)
+			*dvert = mesh->dvert;
+	}
+}
+
 
 /* only called by BKE_modifier.h/modifier.c */
 void modifier_type_init(ModifierTypeInfo *types[])