Re-allows constants in global space to be initialized with function call

[Chaosus]

Allows using built-in functions in global constant initialization (was locked by mistake in #72494). The custom functions should be disallowed from using (lead to shader crash).

shader_type spatial;

const float s = 1.0f / sqrt(2.0f); // correct (error, previously)

void foo() {
return 0.0;
}

const float s2 = 1.0f / foo(); // error (otherwise lead to shader failure)

void fragment() {
}

Also, I've forbidden using custom function call in non-global constant declaration (its non-standard behavior and be restricted on shadertoy/shadered) eg:

void foo(){
return 1.0f;
}

void fragment(){
const float t = 1.0f / foo(); // error now
}

PS: I've added the separate hashmap for functions, so it is easy to check up, the actual change is at line 5211-5217 shader_language.cpp.

[clayjohn]

Looks good to me

Let's not cherrypick as users might be relying on the previous erroneous behaviour (previously users could assign to a local constant using a function and that violates spec)

[akien-mga]

Thanks!