bmesh: corrected fix for svulpting on non-multires mesh

Previous fix wasn't correct. It made things working, but it removed
advantage of using PBVH for speed because derivedMesh used to be
recreated on each stroke step.

This commit fixes non-multires sculpting for Solid shading,
wireframe and render wouldn't work correct -- it'll require
a bit more tricky things because DM created for base mesh
doesn't points to mesh->mverts anymore, so sculpting and
drawing modules are using different verts arrays.

source/blender/blenkernel/intern/cdderivedmesh.c

@@ -239,7 +239,7 @@ static int can_pbvh_draw(Object *ob, DerivedMesh *dm)
 	if(deformed)
 		return 0;
 
-	return (cddm->mvert == me->mvert) || ob->sculpt->kb;
+	return dm->deformedOnly || ob->sculpt->kb;
 }
 
 static struct PBVH *cdDM_getPBVH(Object *ob, DerivedMesh *dm)

source/blender/editors/sculpt_paint/sculpt.c

@@ -3382,18 +3382,13 @@ static void sculpt_flush_update(bContext *C)
 	ARegion *ar = CTX_wm_region(C);
 	MultiresModifierData *mmd = ss->multires;
 
-	if (!ss->cache) {
-		if(mmd)
-			multires_mark_as_modified(ob);
-	}
-
+	if(mmd)
+		multires_mark_as_modified(ob);
 	if(ob->derivedFinal) /* VBO no longer valid */
 		GPU_drawobject_free(ob->derivedFinal);
 
-	if (!ss->cache || !mmd)
-		DAG_id_tag_update(&ob->id, OB_RECALC_DATA);
-	
 	if(ss->modifiers_active) {
+		DAG_id_tag_update(&ob->id, OB_RECALC_DATA);
 		ED_region_tag_redraw(ar);
 	}
 	else {

source/blender/editors/sculpt_paint/sculpt_undo.c

@@ -193,7 +193,7 @@ static void sculpt_undo_restore(bContext *C, ListBase *lb)
 		if((mmd=sculpt_multires_active(scene, ob)))
 			multires_mark_as_modified(ob);
 
-		tag_update= ((Mesh*)ob->data)->id.us > 1 || !mmd;
+		tag_update= ((Mesh*)ob->data)->id.us > 1;
 
 		if(ss->modifiers_active) {
 			Mesh *mesh= ob->data;