The layer would have to additionally specify the x and y crop to control the scrolling speed of the background. Before rendering we readjust the local camera position to get the parallax effect. No big deal ;-)
This issue can be closed because since commit f56c8f0ad82434ca7f68130b64f13047fc3ab5de it's easily possible to adjust the layer offset of any layer inside the game loop and calculate the parallax effect there.
The layer would have to additionally specify the x and y crop to control the scrolling speed of the background. Before rendering we readjust the local camera position to get the parallax effect. No big deal ;-)