Allow passing non-variable constant to const function param in shaders

[Chaosus]

Fix incorrect error when passing values to the function with const parameter:

shader_type spatial;

vec3 test(const float t) {
	return vec3(1, 0, 0);
}

void fragment() {
	test(0.9); // ERROR LINE
}

Also added const parameter processing to gles3 shader compiler.