Increase sign glow size (and other changes that need discussion)

[Kuma-Boo]

Description of changes

• Increases sign glow by one pixel.
• Gave signs their own material.
• Duplicate materials at runtime instead of in editor

Extra notes

Currently every object that uses glow has its own local copy of glow.tres (resource_local_to_scene), so should we ever need to make these kind of changes to an object, we would have to go through every single scene that has said object and manually change it.

Ideally we could use instance shader uniforms, but it doesn't seem to work for canvasitems in 4.1, leaving us with two workarounds:

1. Have glowing objects duplicate their materials at runtime (used in this pr)
2. Brute force it and hope we never have to change anything

Issue(s)

Closes #224

[Koopa1018]

Personally I'm all for the duplicate-at-runtime approach. Currently, having them local to scene generates a LOT of version control noise (their IDs change every time the scene is loaded--a known problem in Godot, hopefully fixed in the future, but not yet).

[Koopa1018]

@GTcreyon @RandomCatDude @jaschutte thoughts on what's being proposed here?

[Koopa1018]

Eh. I kinda doubt this change actually touches any areas outside of signs. Let's get this thing reviewed.

EDIT: Also, having a centralized sign material will make future project-wide material edits much more practical.