GDM login features 3 thin brown lines around right-hand edges

Binary package hint: ubuntu-artwork

The current GDM login artwork appears to have been mis-antialiased and features a 1-pixel dark brown line along the right-hand edge and along the end of the 2 edges closest to it.

This thin line is much dark than the surrounding foreground or desktop background and so remains visible.

Ideally, these 3 sections of line would be removed so that the colour flows straight from the foreground GDM to the background desktop image.