window: fix potential crash with multiple egl windows on windows

I didn't actually see this crash, but the same potential problem
was present, so adjust the code as per 9712d4d

refs: #316

window/src/os/windows/connection.rs

@@ -18,6 +18,8 @@ pub struct Connection {
     event_handle: HANDLE,
     pub(crate) windows: RefCell<HashMap<HWindow, Rc<RefCell<WindowInner>>>>,
     timers: RefCell<HashMap<UINT_PTR, UINT_PTR>>,
+    #[cfg(feature = "opengl")]
+    pub(crate) gl_connection: RefCell<Option<Rc<crate::egl::GlConnection>>>,
 }
 
 impl ConnectionOps for Connection {
@@ -97,6 +99,8 @@ impl Connection {
             event_handle,
             windows: RefCell::new(HashMap::new()),
             timers: RefCell::new(HashMap::new()),
+            #[cfg(feature = "opengl")]
+            gl_connection: RefCell::new(None),
         })
     }
 

window/src/os/windows/window.rs

@@ -132,14 +132,23 @@ impl WindowInner {
     #[cfg(feature = "opengl")]
     fn enable_opengl(&mut self) -> anyhow::Result<()> {
         let window = Window(self.hwnd);
+        let conn = Connection::get().unwrap();
 
         let gl_state = if is_egl_preferred() {
-            crate::egl::GlState::create(None, self.hwnd.0)
+            match conn.gl_connection.borrow().as_ref() {
+                None => crate::egl::GlState::create(None, self.hwnd.0),
+                Some(glconn) => {
+                    crate::egl::GlState::create_with_existing_connection(glconn, self.hwnd.0)
+                }
+            }
         } else {
             Err(anyhow::anyhow!("Config says to avoid EGL"))
         }
         .and_then(|egl| unsafe {
             log::error!("Initialized EGL!");
+            conn.gl_connection
+                .borrow_mut()
+                .replace(Rc::clone(egl.get_connection()));
             let backend = Rc::new(egl);
             Ok(glium::backend::Context::new(
                 backend,