[msl-out] use back::COMPONENTS (#2124)

src/back/msl/writer.rs

@@ -1442,7 +1442,7 @@ impl<W: Write> Writer<W> {
                         if offset.is_none() && !is_cube_map {
                             write!(self.out, ", {}::int2(0)", NAMESPACE)?;
                         }
-                        let letter = ['x', 'y', 'z', 'w'][component as usize];
+                        let letter = back::COMPONENTS[component as usize];
                         write!(self.out, ", {}::component::{}", NAMESPACE, letter)?;
                     }
                 }