Make WindowBuilder and HeadlessRendererBuilder derive Clone.

src/platform/linux/api_dispatch.rs

@@ -22,7 +22,7 @@ use api::x11::XConnection;
 use api::x11::XError;
 use api::x11::XNotSupported;
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes;
 
 enum Backend {

src/platform/linux/mod.rs

@@ -15,7 +15,7 @@ pub use self::api_dispatch::{WaitEventsIterator, PollEventsIterator};
 pub use self::api_dispatch::PlatformSpecificWindowBuilderAttributes;
 mod api_dispatch;
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificHeadlessBuilderAttributes;
 
 pub struct HeadlessContext(OsMesaContext);