Fix shader crash when using a varying in separate func before it defined

[Chaosus]

This will fix the shader crash, when the function used the varying before it's defined:

shader_type spatial;

varying vec3 foo;

vec3 function1() {
	return foo;
}

void vertex() {
	vec3 v = function1(); // BUG
}

void fragment() {
	foo = vec3(1, 0, 1); // Fragment-to-light varying.
}

I've also

• Fix Shader: can't use varyings as read-only parameters in custom functions #70434

[Chaosus]

Note: by "crash" here, I mean the shader compiler failure with long list of errors, not an actual crash :)

[akien-mga]

Thanks!