Mass: add a missing variable to CustomStyle.

src/Mass/Mass.h

@@ -52,6 +52,7 @@ struct CustomStyle {
     Int patternId = 0;
     Float opacity = 0.0f;
     Float offsetX = 0.0f;
+    Float offsetY = 0.0f;
     Float rotation = 0.0f;
     Float scale = 0.0f;
 };