Add const qualifier support for function arguments in shaders (3.x)

[Calinou]

3.x version of #40010. I found out about this while skimming over commits for writing the 4.0 alpha changelog 🙂

Example:

void test(const int t) {}

shader_type spatial;

vec3 test_function_mutable(float red) {
	red = 0.0;
	return vec3(red, 0.3, 0.3);
}

vec3 test_function_const(const float red) {
	//red = 0.0; // This causes an error if uncommented.
	return vec3(red, 0.3, 0.3);
}

void fragment() {
	ALBEDO = test_function_const(0.9);
}

[Chaosus]

@Chaosus The following code gives me an error on both master and 3.x, is this expected?

Seems like it's bugged and it's allowed in GLSL.

@Calinou You can add the changes from #55113 (shader_language.cpp) to fix that.

[Calinou]

@Calinou You can add the changes from #55113 (shader_language.cpp) to fix that.

Done 🙂 I tested the PR again on both GLES3 and GLES2.

[akien-mga]

Thanks!