Fix selected image is modified by moving the pivot point in certain positions (fix #3901)

Prior to this fix, there was a spurious movement on a selected image
simply by moving the pivot point. Steps to reproduce it:

- Select a canvas area
- Zoom at 300%
- Make the pivot visible and move it to a negative coordinate
  position (less than the origin 0,0)
- If you rotate/skew the image this problem cannot be reproduced

src/app/ui/editor/pixels_movement.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2019-2021  Igara Studio S.A.
+// Copyright (C) 2019-2023  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -699,7 +699,7 @@ void PixelsMovement::moveImage(const gfx::PointF& pos, MoveModifier moveModifier
 
     case PivotHandle: {
       // Calculate the new position of the pivot
-      gfx::PointF newPivot = m_initialData.pivot() + pos - m_catchPos;
+      gfx::PointF newPivot = m_initialData.pivot() + gfx::Point(pos) - m_catchPos;
       newTransformation = m_initialData;
       newTransformation.displacePivotTo(newPivot);
       break;