Prevent redefinition of main functions in shader

[Chaosus]

Prevent writing such code:

shader_types spatial;

void fragment() {
}

void fragment() {
}

Note, that due to vertex/fragment/light technically are not functions, but just code fragments inserted into the main function parser must not prevent writing code like this:

void fragment() {
}

const vec3 fragment = vec3(1, 0, 0);

so I did not modify a _find_identifier for this.

[akien-mga]

Thanks!