Mirror option is not working

[i30817]

At least for random pngs.

testcase:

#! /usr/bin/env python3

from PIL import Image
import subprocess, io

solid_black = Image.new('RGBA', (50,50), (0, 0, 0, 255))
solid_red = Image.new('RGBA', (50,50), (255, 0, 0, 255))

combined = Image.new('RGBA', (100, 50))
combined.paste(solid_black, box=(0,0))
combined.paste(solid_red, box=(solid_black.size[0],0))

#for comparasion, original
combined.show()
#for bug view
with io.BytesIO() as f:
    combined.save(f, format='png')
    #supposed to be reversed
    subprocess.run(['viu', '-m', '-'], input=f.getbuffer())