Wayland: Fix resize events before ack_configure

The surface was resized and the size event was emitted before we had
sent xdg_surface::ack_configure.  If user code then called some GLFW
function that commited the surface, those changes would all get applied
to the wrong configure event.

This postpones size changes until after the ack.

(cherry picked from commit e33db6d7aa)

src/wl_platform.h

@@ -189,6 +189,10 @@ typedef struct _GLFWwindowWayland
     struct wl_egl_window*       native;
     struct wl_callback*         callback;
 
+    struct {
+        int                     width, height;
+    } pending;
+
     struct {
         struct xdg_surface*     surface;
         struct xdg_toplevel*    toplevel;