fixed bug on windows

ahb_cmd_render.py

@@ -200,6 +200,7 @@ class AHB_Render:
                 bg = Image.new('RGB', resolution, (255,255,255))
                 if prev_stage_id is not None:
                     prev_stage = Image.open(dir + "/stages/" + stage_name + "-bg.png")
+                    prev_stage.load()
                     os.remove(dir + "/stages/" + stage_name + "-bg.png")
                     bg.paste(prev_stage, None, prev_stage.getchannel('A'))
                 fg = Image.open(dir + "/stages/" + stage_name + ".png")