Add Window::set_transparent

Provide a hint to system compositor whether the window is transparent or
not. Only implemented on macOS and Wayland for now.

src/platform_impl/linux/mod.rs

@@ -324,6 +324,11 @@ impl Window {
         x11_or_wayland!(match self; Window(w) => w.set_title(title));
     }
 
+    #[inline]
+    pub fn set_transparent(&self, transparent: bool) {
+        x11_or_wayland!(match self; Window(w) => w.set_transparent(transparent));
+    }
+
     #[inline]
     pub fn set_visible(&self, visible: bool) {
         x11_or_wayland!(match self; Window(w) => w.set_visible(visible))