MapEditor: Allow you to modify opacity of materials

[LukeSaward]

1. Get material from static mesh

UPrimitiveComponent::GetMaterial(int ElementIndex)

2. Create dynamic material instance

UMaterialInstanceDynamic::Create(UMaterialInterface* ParentMaterial, UObject* InOuter)

3. Set scalar parameter value of the material

let fun2: extern_fn!(fn( this: *const c_void, name: FName, value: f32 ))
    = unsafe { mem::transmute(UMATERIALINSTANCEDYNAMIC_SETSCALARPARAMETERVALUE.load(Ordering::SeqCst)) };
fun2(obj.as_ptr() as *const c_void, FName::from("opacity"), 0.3);

4. Set mesh's material to the dynamic material

unsafe {
    let fun2376 = mesh.class().find_function("SetMaterial").unwrap();
    let params = fun2376.create_argument_struct();
    let thingy23 = ObjectWrapper::new(thing as *mut UObject);
    params.get_field("Material").set_object(&thingy23);
    fun2376.call(mesh.as_ptr(), &params);
}

Ties into #301