Godot: Implement rotation for parallax background

[m93a]

Godot version: 3.1.1 (and 3.2 ad @fossegutten)
OS/device including version: Ubuntu 19.04

Issue description:
Parallax background ignores the rotation of the camera. Instead all the layers should rotate with the rotation center being in the middle of the screen. (All layers with the same speed, disregarding the motion scaling.)

[fossegutten]

This seems to be a bug. The parallax backgrounds actually offset horizontally and vertically, instead of rotating, when camera2d is rotated. This is magnified the further away camera2d moves from zero position.
Gif and reproduction project is in my duplicate issue #35782