Add PSRDNoise implementations for 2D and 3D

[hmans]

This PR adds 2D and 3D versions of the excellent PSRDNoise algorithm.

Notes:

• The code is an almost 1:1 copy of the original psrdnoise2-min.glsl and psrdnoise3-min.glsl files. Only the mat3 initialization in psrdnoise3 had to be changed to match Godot's preferred matrix instantiation signature.
• I've tested both functions and can confirm that everything is working.
• In the original files, the functions were always called psrdnoise. I've renamed them to psrdnoise2 and psrdnoise3 so both can be imported without conflict.
• Both functions allow for a final out vec2/3 gradient parameter. This is a very useful, but purely optional feature, but the signature will always require the user to provide a variable here, even if they don't use it, which is a little bit annoying. I'm not sure if Godot's shading language allows for making parameters optional? If not, I could provide additional overloads that do away with the parameter. Please let me know what you think.
  • After finding out that Godot's shader language doesn't support function overloading, I've pushed another commit that provides psrdnoise3 and psrdnoise3_with_gradient (and same for 2) functions.
• Similarly, psrdnoise3.gdshaderinc exports a permute function. The potential for namespace collisions (especially if you later want to add other noise implementations) is high. How should we proceed here? We should probably rename the function to something a little less generic — anything else?
  • I've renamed it to __psrdnoise3_permute. If there is anything else we should do with it, let me know.

[paddy-exe]

If the comment lines inside the shader code are removed and one blank line at the end of each file I can merge this :D thank you!

[paddy-exe]

Awesome work! Thanks :D