Add basic support to evaluate operator value in shader language

[Chaosus]

This PR adds a basic set of evaluator functions to the shader language. This might be needed in some places, like an array initialization. After this PR got merged, user may define and use basic constant expression like:

shader_type spatial;

const int ARRAY_SIZE = 1 + (9 / 3); // evaluates to 4

void fragment() { 
	int array[ARRAY_SIZE] = {0, 0, 0, 0}; // correct
}

Also, I've fixed parse_array_size function to allow declaring expression inside it:

shader_type spatial;

void fragment() {
	int array[1 + 2 / 2] = {0, 0}; // correct
}

Also, constant may be used like this:

shader_type spatial;

const int SIZE = 3;

void fragment() {
	int array[SIZE + SIZE] = {0, 0, 0, 0, 0, 0}; // correctly evaluates to 6
}

Besides array size, the expressions can now be passed to specific built-in functions:

const int SIZE = 1;

textureGather(sampler, vec2(0.0), SIZE + 1); // OK
textureGather(sampler, vec2(0.0), SIZE + 4); // FAILED (must be within 0..3 range)

Note, the advanced constant constructions like:

const int ARRAY_SIZE = ivec2(1, 0).x;

void fragment() {
	int array[ARRAY_SIZE] = {0};
}

still doesn't work, I may improve it lately.

• Closes Can't use calculated constant as array size in shader #72087

PS: Yes, I renamed ShaderLanguage::ConstantNode::Value to ShaderLanguage::Scalar because it fits better and other nodes may use it, not only constant node (currently an OperatorNode will also start to use it).

[Calinou]

Tested locally, it works as expected in all rendering methods. Thanks for improving the error reporting on mismatched array sizes too 🙂

shader_type canvas_item;

void fragment() {
	const int SIZE = 1 + 2;
	int items[SIZE] = { 1, 2, 3 };

	int items2[SIZE + 1] = { 1, 2, 3, 4 };
	int items3[SIZE * 2] = { 1, 2, 3, 4, 5, 6 };

	COLOR.gb *= float(items3[5]) * 0.1;

	// Not valid, as it needs to be a constant.
	//int dyn = 3;
	//int items_dyn[SIZE + dyn] = { 1, 2, 3, 4, 5, 6 };
}

[akien-mga]

Thanks!

[geekley]

If I understand correctly, I assume the code in this PR may be useful to solve #96504 as well... can I get a confirmation?

[Chaosus]

@geekley Well, technically preprocessor and shader language are in separate files and in order to work together they need to use a single header and all that evaluations needs to be done in it. Need to spend another several hours for that..