Fix swimming on slopes

src/collision/collision_system.cpp

@@ -675,22 +675,24 @@ CollisionSystem::is_free_of_tiles(const Rectf& rect, const bool ignoreUnisolid,
       for (int y = test_tiles.top; y < test_tiles.bottom; ++y) {
         const Tile& tile = solids->get_tile(x, y);
 
-        if (tile.get_attributes() & tiletype)
-          return false;
-        if (tile.is_unisolid() && !ignoreUnisolid)
-          return false;
+        if (!(tile.get_attributes() & tiletype))
+          continue;
+        if (tile.is_unisolid () && ignoreUnisolid)
+          continue;
         if (tile.is_slope ()) {
           AATriangle triangle;
           const Rectf tbbox = solids->get_tile_bbox(x, y);
           triangle = AATriangle(tbbox, tile.get_data());
           Constraints constraints;
-          if (collision::rectangle_aatriangle(&constraints, rect, triangle))
-            return false;
+          if (!collision::rectangle_aatriangle(&constraints, rect, triangle))
+            continue;
         }
+        // We have a solid tile that overlaps the given rectangle.
+        return false;
       }
     }
   }
-  
+
   return true;
 }