Relative sprite scaling (pythonarcade#492)

* Add relative rescale method to Sprite

* Add rescale method to SpriteList

arcade/sprite.py

@@ -433,6 +433,12 @@ def _set_scale(self, new_value: float):
 
     scale = property(_get_scale, _set_scale)
 
+    def rescale_relative_to_point(self, point: Point, factor: float) -> None:
+        """ Rescale the sprite relative to a different point than its center. """
+        self.scale *= factor
+        self.center_x = (self.center_x - point[0]) * factor + point[0]
+        self.center_y = (self.center_y - point[1]) * factor + point[1]
+
     def _get_center_x(self) -> float:
         """ Get the center x coordinate of the sprite. """
         return self._position[0]

arcade/sprite_list.py

@@ -359,6 +359,19 @@ def update_animation(self, delta_time: float = 1/60):
         for sprite in self.sprite_list:
             sprite.update_animation(delta_time)
 
+    def _get_center(self) -> Tuple[float, float]:
+        """ Get the mean center coordinates of all sprites in the list. """
+        x = sum((sprite.center_x for sprite in self.sprite_list)) / len(self.sprite_list)
+        y = sum((sprite.center_y for sprite in self.sprite_list)) / len(self.sprite_list)
+        return x, y
+
+    center = property(_get_center)
+
+    def rescale(self, factor: float) -> None:
+        """ Rescale all sprites in the list relative to the spritelists center. """
+        for sprite in self.sprite_list:
+            sprite.rescale_relative_to_point(self.center, factor)
+
     def move(self, change_x: float, change_y: float):
         """
         Moves all Sprites in the list by the same amount.