corrections to modifiers from recent normal handling changes

- solidify didn't define a dependsOnNormals callback (which it should have)
- build wasn't passing on dirty normals.
- decimate wasnt setting dirty normals.

source/blender/modifiers/intern/MOD_build.c

@@ -296,6 +296,10 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *UNUSED(ob),
 	MEM_freeN(edgeMap);
 	MEM_freeN(faceMap);
 
+	if (dm->dirty & DM_DIRTY_NORMALS) {
+		result->dirty |= DM_DIRTY_NORMALS;
+	}
+
 	return result;
 }
 

source/blender/modifiers/intern/MOD_decimate.c

@@ -197,6 +197,8 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob,
 	TIMEIT_END(decim);
 #endif
 
+	result->dirty = DM_DIRTY_NORMALS;
+
 	return result;
 }
 

source/blender/modifiers/intern/MOD_solidify.c

@@ -819,7 +819,7 @@ static DerivedMesh *applyModifier(
 		MEM_freeN(old_vert_arr);
 
 	/* must recalculate normals with vgroups since they can displace unevenly [#26888] */
-	if (dvert) {
+	if ((dm->dirty & DM_DIRTY_NORMALS) || dvert) {
 		result->dirty |= DM_DIRTY_NORMALS;
 	}
 
@@ -832,6 +832,12 @@ static DerivedMesh *applyModifier(
 
 #undef SOLIDIFY_SIDE_NORMALS
 
+static bool dependsOnNormals(ModifierData *md)
+{
+	SolidifyModifierData *smd = (SolidifyModifierData *) md;
+
+	return (smd->flag & MOD_SOLIDIFY_NORMAL_CALC) == 0;
+}
 
 ModifierTypeInfo modifierType_Solidify = {
 	/* name */              "Solidify",
@@ -858,7 +864,7 @@ ModifierTypeInfo modifierType_Solidify = {
 	/* isDisabled */        NULL,
 	/* updateDepgraph */    NULL,
 	/* dependsOnTime */     NULL,
-	/* dependsOnNormals */  NULL,
+	/* dependsOnNormals */  dependsOnNormals,
 	/* foreachObjectLink */ NULL,
 	/* foreachIDLink */     NULL,
 	/* foreachTexLink */    NULL,