amend! WIP: fix(msl-out): don't check member.binding in write_type_defs, should_pack_struct_member

fix(msl-out): emit and init `struct` member padding always

Resolves
[`gfx-rs/wgpu`#4701](#4701).

Author: ErichDonGubler

naga/src/back/msl/writer.rs

@@ -4267,6 +4267,13 @@ impl<W: Write> Writer<W> {
                             if member_index != 0 {
                                 write!(self.out, ", ")?;
                             }
+                            // insert padding initialization, if needed
+                            if self
+                                .struct_member_pads
+                                .contains(&(arg.ty, member_index as u32))
+                            {
+                                write!(self.out, "{{}}, ")?;
+                            }
                             if let Some(crate::Binding::Location { .. }) = member.binding {
                                 write!(self.out, "{varyings_member_name}.")?;
                             }