Painting side texture doesn't match the frame texture

The bug:

All painting variants have different frames, but the side of the painting entity uses the back.png texure which doesn't match the frame of the painting.

Steps on how to reproduce:

1. Equip barrier block, painting

2. Build a 4x4 wall with barriers

3. Place a painting on the wall

4. Look at the sides of the painting

Painting sides:

Fix: