Use std::swap to swap double instead of using temp variable

Author: joto

src/expire-tiles.cpp

@@ -173,9 +173,7 @@ void expire_tiles::from_line(geom::point_t const &a, geom::point_t const &b)
            but for simplicity, treat the coordinates as a bounding box
            and expire everything within that box. */
         if (y1 > y2) {
-            double const temp = y2;
-            y2 = y1;
-            y1 = temp;
+            std::swap(y1, y2);
         }
         for (int x = x1 - tile_expiry_leeway; x <= x2 + tile_expiry_leeway;
              ++x) {