Fix StyleBoxFlat rectangles skewing independently

(cherry picked from commit 86c773d208afee2b37d3c2f595f4e7d24c78e2cb)

scene/resources/style_box_flat.cpp

@@ -300,8 +300,8 @@ inline void draw_rounded_rectangle(Vector<Vector2> &verts, Vector<int> &indices,
 
 				const real_t x = radius * (real_t)cos((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.x;
 				const real_t y = radius * (real_t)sin((corner_index + detail / (double)adapted_corner_detail) * (Math_TAU / 4.0) + Math_PI) + corner_point.y;
-				const float x_skew = -skew.x * (y - ring_rect.get_center().y);
-				const float y_skew = -skew.y * (x - ring_rect.get_center().x);
+				const float x_skew = -skew.x * (y - style_rect.get_center().y);
+				const float y_skew = -skew.y * (x - style_rect.get_center().x);
 				verts.push_back(Vector2(x + x_skew, y + y_skew));
 				colors.push_back(color);
 			}