Don't set the background color when initializing with transparency (#3657)

Setting the background color changes how the window title bar appears,
which is something that the application should customize itself if it
wants this behaviour (and also, it wasn't set when calling
`set_transparent`, so the behaviour wasn't consistent).

src/window.rs

@@ -941,6 +941,8 @@ impl Window {
     ///
     /// ## Platform-specific
     ///
+    /// - **macOS:** If you're not drawing to the window yourself, you might have to set the
+    ///   background color of the window to enable transparency.
     /// - **Web / iOS / Android:** Unsupported.
     /// - **X11:** Can only be set while building the window, with
     ///   [`WindowAttributes::with_transparent`].