Fix don't paint transparent color via Eraser tool in Background layers (

fix aseprite#4370)

Before this fix, when working with sprites that included a
background layer and using the Eraser tool, drawing transparent
pixels was allowed if the secondary color was transparent +
right click (i.e. eraser in replace mode "Primary color ->to->
Secondary").
Original issue aseprite#4370 description:
"Downloaded PNG in RGB mode fails to support transparency: erase
uses secondary color and export PNG replaces transparent color with
black"

src/app/tools/inks.h

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2018-2022 Igara Studio S.A.
+// Copyright (C) 2018-2024 Igara Studio S.A.
 // Copyright (C) 2001-2018 David Capello
 //
 // This program is distributed under the terms of
@@ -366,7 +366,8 @@ class EraserInk : public BaseInk {
  }
  case ReplaceFgWithBg:
  loop->setPrimaryColor(loop->getFgColor());
- loop->setSecondaryColor(loop->getBgColor());
+ loop->setSecondaryColor(loop->getBgColor() |
+ (loop->getLayer()->isBackground() ? ColorAMask : 0));
  setProc(get_ink_proc2<ReplaceInkProcessing>(loop));
  break;
 
@@ -516,7 +517,7 @@ class SelectionInk : public BaseInk {
  m_mask.unfreeze();
 
  loop->setMask(&m_mask);
- double cornerThick = (loop->isTilemapMode()) ? 
+ double cornerThick = (loop->isTilemapMode()) ?
  CORNER_THICK_FOR_TILEMAP_MODE :
  CORNER_THICK_FOR_PIXELS_MODE;
  loop->getDocument()->setTransformation(