Fix shader failure when using non-const initializer on a constant

[Chaosus]

Prevent using non-const (or uniform) expression in constant initializers:

uniform float scalar_uniform;

const float scalar = test; // INCORRECT

uniform float scalar_uniform;
const float scalar_constant = 1.0;

const vec3 v= vec3(0, 0, scalar_uniform); // INCORRECT
const vec3 v2= vec3(0, 0, scalar_constant); // CORRECT

That will also prevent using them in a local constants:

uniform float scalar_uniform;

void fragment() {
	const vec3 v= vec3(0, 0, scalar_uniform); // INCORRECT
}

That would lead to a compatibility breakage, because shaders like this seems working correctly (at least on Vulkan renderer) - but, for example, SHADERed accept it as error:

• Fix Casting a uniform bool to an integer or float breaks the renderer in GLSL shaders #72466

[clayjohn]

Looks good to me!

[akien-mga]

Thanks!