Make WindowBuilder and HeadlessRendererBuilder derive Clone.

src/api/android/mod.rs

@@ -65,9 +65,9 @@ impl MonitorId {
     }
 }
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes;
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificHeadlessBuilderAttributes;
 
 pub struct PollEventsIterator<'a> {

src/api/cocoa/headless.rs

@@ -13,7 +13,7 @@ use cocoa::appkit::*;
 use PixelFormat;
 use api::cocoa::helpers;
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificHeadlessBuilderAttributes;
 
 pub struct HeadlessContext {

src/api/cocoa/mod.rs

@@ -181,7 +181,7 @@ impl Drop for WindowDelegate {
     }
 }
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes {
     pub activation_policy: ActivationPolicy,
 }

src/api/ios/mod.rs

@@ -177,7 +177,7 @@ impl MonitorId {
     }
 }
 
-#[derive(Default)]
+#[derive(Clone, Default)]
 pub struct PlatformSpecificWindowBuilderAttributes;
 
 impl Window {