Fullscreen setting is broken

[Anutrix]

Godot version:
master 321ce4d

OS/device including version:
Windows 10 Pro

Issue description:
Setting the fullscreen option just makes the window of the game borderless. The size remains same and doesn't cover the whole screen.
Fullscreen unchecked:

Fullscreen checked:

Steps to reproduce:

1. Check fullscreen option in Project Settings.
2. Run the project.

Minimal reproduction project:
Any

[Anutrix]

Before:
After Shift+F11:

The issue is that input still works and it works on the window behind it. Can wreak havoc. Should be high-risk issue.

[KoBeWi]

For some reason this still exists in 9e34ba4 (although the PR did fix it, so it just broke again)