Fix incorrect padding between frame images when packing sprite sheets (…

…fix aseprite/aseprite#3993)

Prior to this fix, Export Sprite Sheet incorrectly generated a
shape padding in some frame image when the following options
were chosen:
- Packed sheet type
- Trim Cels
- Spacing > 0

gfx/packing_rects.cpp

@@ -1,5 +1,5 @@
 // LAF Gfx Library
-// Copyright (C) 2019  Igara Studio S.A.
+// Copyright (C) 2019-2023  Igara Studio S.A.
 // Copyright (C) 2001-2014 David Capello
 //
 // This file is released under the terms of the MIT license.
@@ -112,14 +112,17 @@ bool PackingRects::pack(const Size& size,
         if (token.canceled())
           return false;
 
-        gfx::Rect possible(m_bounds.x + u, m_bounds.y + v, rc.w, rc.h);
+        gfx::Rect possible(m_bounds.x + u,
+                           m_bounds.y + v,
+                           rc.w + m_shapePadding,
+                           rc.h + m_shapePadding);
         Region::Overlap overlap = rgn.contains(possible);
         if (overlap == Region::In) {
-          rc = possible;
           rgn.createSubtraction(
             rgn,
-            gfx::Region(Rect(rc).inflate(m_shapePadding))
+            gfx::Region(possible)
           );
+          rc = possible.inflate(-m_shapePadding);
           goto next_rc;
         }
       }