bindgen: enable derive_default

That's nice!!

bindgen option of rustified_enum or newtype_enum are NOT enabled:
* newtype_enum: They can't be created from u32, so we can't make our fancy enums with rustic case.
* rusitified_enum: They're dangerous if we try to match them: rust-lang/rust#36927
* default: Enum variants are defined as constants and FFI (fields and parameters) will be weakly typed. To get strictly typed, we need to wrap them into our `enum`(s).

rokol_ffi/build.rs

@@ -104,6 +104,7 @@ fn make_sokol(wrapper: &str, ffi_file: &str) {
         let b = b.header(&wrapper);
         let b = b.clang_arg(format!("-I{}", sokol_dir.display()));
         // let b = renderer.set_bindgen_flag(b);
+        let b = b.derive_default(true);
         b.generate().unwrap()
     };