Window width affects directional shadow resolution (fixed in `master`)

[EzraT]

Godot version:
Affects both 3.0.6 and Official 3.1 Alpha 2

OS/device including version:
Linux Mint 19 - 64bit
Nvidia GeForce GTX 1060 on driver version: 396.54

Issue description:
For some reason the directional shadow resolution decreases as the window width increases, and increases as the width decreases.
Happens in the editor window and also in a running game.
Video

I'm assuming this isn't intended behaviour?
OmniLight and SpotLight do not seem affected.

Steps to reproduce:

1. Set up a scene with meshes and a Directional Light with shadows enabled.
2. Decrease and increase the width of the 3D scene window and observe shadows.

[Calinou]

This is already mentioned in #12148 (comment) and #13575 (comment), but since there doesn't appear to be a dedicated issue for this bug, I'll leave this one open.

[EzraT]

Any chance this one can be added to the 3.1 milestone?
The quality difference is quite dramatic.

[and3rson]

Bumping as this issue still persists.

[Calinou]

I'm not sure if this can be resolved completely (any ideas?). Still, we could make it so the shadow quality becomes worse when the viewport becomes taller, instead of wider. This would play better with ultrawide displays which are becoming more common.

[Calinou]

I tested how shadow quality changes when the camera is set to use Keep Width (Vert-) aspect instead of Keep Height (Hor+, the default). The shadow quality still decreases as the viewport becomes wider (even though the horizontal FOV doesn't increase, causing the image to look "zoomed in"), but it doesn't decrease as the viewport becomes taller.

This may hint at a configuration issue somewhere in the rendering code or shader.