Paratroopas make version control noise

[Koopa1018]

Describe the bug
Since the 4.1 update, Paratroopas' per-scene materials generate a new UID every time a level is opened containing them. These UID changes show up in version control, but are also completely useless and distract from actually-important changes being made.

To Reproduce

1. In the Godot editor, open any level with Paratroopas in it.
2. Save the level, without even making changes.
3. View the level in your Git client of choice.

Expected behavior
Level files only appear in version control when intentional changes were made.

Screenshots

System Information

• OS/Browser: Windows 10
• Game version: 4.1 branch
• Other specs: VCS is GitHub Desktop

Additional context
Possibly solve in similar way to #227?

[Tricia]

I'll take a look at this.