Fixed styles for undecorated windows on Windows.

src/win32_window.c

@@ -788,10 +788,7 @@ static int createWindow(_GLFWwindow* window,
             }
         }
         else
-        {
-            window->win32.dwStyle = WS_POPUP;
-            window->win32.dwExStyle = 0;
-        }
+            window->win32.dwStyle |= WS_POPUP;
 
         xpos = CW_USEDEFAULT;
         ypos = CW_USEDEFAULT;