correct signature generation of non-memory enum writer

Signed-off-by: Elias Naur <mail@eliasnaur.com>

piet-gpu-derive/src/glsl.rs

@@ -536,7 +536,7 @@ fn gen_enum_write(
                 } else {
                     writeln!(
                         r,
-                        "void {}_{}_write(Alloc a, {}Ref ref, {} s) {{",
+                        "void {}_{}_write({}Ref ref, {} s) {{",
                         name, var_name, name, structname
                     )
                     .unwrap();