varying of type `int`, `uint`, `bool` and vector types are disallowed even if declared `flat`

[lyuma]

Godot version

4.0.alpha

System information

Windows 10

Issue description

Godot should allow any primitive data type other than sampler2D to be passed as varying.

shaders do not allow interpolating non-floating point values. For this reason, non-float varyings are required to be flat. We could do this automatically, or we could display an error message and ask the user to add flat.

See also: #45918 , but allow for all of vertex, fragment and light.

I am in favor of automatically assuming flat if not a floating point type.

Steps to reproduce

1. Create a new MeshInstance3D
2. Create a new StandardMaterial3D and convert to shader material.
3. Declare varying flat int instance_id;
4. Set instance_id = INSTANCE_ID; in vertex()
5. it shows error "Invalid type for varying, only 'float', ..."

Minimal reproduction project

No response

[Calinou]

According to @clayjohn, even GLES2 should allow this. So this change can be made in 3.x too, in both GLES3 and GLES2.