Clarify the usage of NORMAL_MAP in gdshader

[Raildex]

Your Godot version:
4.6-1

Issue description:
The documentation says:
Set normal here if reading normal from a texture instead of NORMAL.
This is insufficient because there are multiple parameters to take into account:

You can have two kinds of textures: RGTC for red/green compressed normal maps or (DXT5/BC3) for "purpleish" tangent space textures.

It's also not clear in which space they are supposed to be in. Is it view space? world space?

According to my tests, Godot expects Red/Green inside NORMAL_MAP, as seen here:

URL to the documentation page (if already existing):
https://docs.godotengine.org/en/stable/tutorials/shaders/shader_reference/spatial_shader.html#doc-spatial-shader

[AThousandShips]

Has to do with this I believe, it should be used with normals that are defined by a texture instead of as a normal normal vector

[Calinou]

You can have two kinds of textures: RGTC for red/green compressed normal maps or (DXT5/BC3) for "purpleish" tangent space textures.

You only need to read the red and green channels (X/Y). The blue channel is ignored, regardless of whether you use RGTC or not.

The engine reconstructs the blue channel in the shader, so it never needs to read the normal map's blue channel even if the source texture contains one.

It's also not clear in which space they are supposed to be in. Is it view space? world space?

Material normal maps are in tangent space, so not view space or world space. To set vertex normals, use NORMAL in vertex() instead.