Bugfix: don’t overwrite the original size of floating windows when changing border style

fixes #1263

include/util.h

@@ -57,6 +57,7 @@ int min(int a, int b);
 int max(int a, int b);
 bool rect_contains(Rect rect, uint32_t x, uint32_t y);
 Rect rect_add(Rect a, Rect b);
+Rect rect_sub(Rect a, Rect b);
 
 /**
  * Returns true if the name consists of only digits.