Add most useful blending mode shaders

[thechayed]

This is a request to contribute, just to add a bunch of Shaders that I create while working with Godot to simplify things for others looking for Godot Shaders in the future, rather than having to refer to multiple repositories.

They will mostly be pretty simple, starting with Overlay:
Left: Original Brick / Right: Gloss Sprite with Overlay Shader added

[NathanLovato]

Why not. If you're going to create multiple simple shaders, I think it'd be great if you could make a gallery-like demo in the Demos/ directory showcasing them all in action on a single screen. Or at least by theme. For instance all blending modes on a single demo.

But the contribution would be most welcome, thanks.