Shadows from DirectionalLight become jittery further away.

[Gyrth]

Godot version:
3.1.1

OS/device including version:
Ubuntu 18.04
Intel® Core™ i5-6600K CPU @ 3.50GHz × 4
GeForce GTX 1060 6GB/PCIe/SSE2

Issue description:
Our scene is rather large and I've noticed that the shadows are quite jittery round the edges. I've narrowed it down to it being caused by the large offset to the zero point. It does not seem to matter how many objects there are in the scene, just that the object is very far removed from the 0,0,0 point. While the in-game camera or editor camera is moving you will see a jittery border around the shadows.

Steps to reproduce:
Run BugReport project.
The animation shows the shadows at both close and far away.

Minimal reproduction project:
BugReport.zip