It's not a problem of mirroring. It also happens in unmirrored images. It's only noticeable in very small images that are scaled up.

What we could theoretically do is adding a panel flag for not rendering the last pixel. You'll then get panels that are 1 pixel too small. But I do not like this solution - it's better to compensate this in the image. Or write a small function that reduces the panel size_x and size_y by 1 pixel.