interiore manager: use image given at init

opencombat/gui/base.py

             'game.building.draw_interior_gap',
             2,
         )
-        self.interior_manager = InteriorManager(TileMap(
-            layer_manager.middleware.get_map_file_path(),
+        self.background_image = Image.open(os.path.join(
+            self.layer_manager.middleware.map_dir_path,
+            'background.png',
         ))
+        self.interior_manager = InteriorManager(
+            TileMap(layer_manager.middleware.get_map_file_path()),
+            original_image=self.background_image,
+        )
         self.map_tile_width = self.layer_manager.middleware.get_cell_width()
         self.map_tile_height = self.layer_manager.middleware.get_cell_height()
 
             interiors = self.interior_manager.get_interiors(
                 where_positions=subject_grid_positions)
 
+            # FIXME BS 2018-01-25: if not, put original background image
             if interiors:
-                image = Image.open(os.path.join(
-                    self.layer_manager.middleware.map_dir_path,
-                    'background.png',
-                ))
                 image_fake_file = io.BytesIO()
-                self.interior_manager.update_image_for_interiors(
-                    image,
+                new_background_image = self.interior_manager.update_image_for_interiors(
                     interiors,
                     self.map_tile_width,
                     self.map_tile_height,
                 )
-                image.save(image_fake_file, format='PNG')
+                new_background_image.save(image_fake_file, format='PNG')
                 self.background_sprite.image = pyglet.image.load(
                     'new_background.png',
                     file=image_fake_file,

opencombat/simulation/interior.py

     def __init__(
         self,
         map_: TMXMap,
+        original_image: PngImageFile,
         configuration: InteriorMapConfiguration=None,
     ) -> None:
         self.interiors = []
         self.map = map_
+        self.original_image = original_image
         self.configuration = configuration or InteriorMapConfiguration()
         self.interiors = self._compute_interiors()
 
 
     def update_image_for_interiors(
         self,
-        image: PngImageFile,
         interiors: typing.List[typing.List[typing.Tuple[int, int]]],
         tile_width: int,
         tile_height: int,
         invert_y: bool=True,
-    ) -> None:
+    ) -> PngImageFile:
         # TODO BS 20171213: Optimization can be done: keep in cache modifications on image instead change it entirely
+        image = self.original_image.copy()
+        image_height = image.height
         pixels = image.load()
 
         for interior in interiors:
 
                         real_y = y
                         if invert_y:
-                            real_y = image.height - 1 - y
+                            real_y = image_height - 1 - y
 
                         pixels[x, real_y] = (0, 0, 0, 0)
+
+        return image