Wayland: Cleanup

Make Wayland code use 'native' in the same senses as the rest of GLFW.

src/wl_platform.h

@@ -233,9 +233,12 @@ typedef struct _GLFWwindowWayland
     GLFWbool                    hovered;
     GLFWbool                    transparent;
     struct wl_surface*          surface;
-    struct wl_egl_window*       native;
     struct wl_callback*         callback;
 
+    struct {
+        struct wl_egl_window*   window;
+    } egl;
+
     struct {
         int                     width, height;
     } pending;