wayland: Avoid excessive exposure events during interactive resizes

Only send the unblocking exposure event once per frame, so that clients using an event watcher won't redraw excessively.

Also ensure that the unblocking exposure is always sent on the libdecor path.

src/video/wayland/SDL_waylandwindow.c

@@ -870,10 +870,10 @@ static void handle_xdg_surface_configure(void *data, struct xdg_surface *xdg, ui
         wind->pending_config_ack = false;
         ConfigureWindowGeometry(window);
         xdg_surface_ack_configure(xdg, serial);
-    } else {
+    } else if (!wind->pending_config_ack) {
         wind->pending_config_ack = true;
 
-        // Send an exposure event so that clients doing deferred updates will trigger a frame callback and make guaranteed forward progress when resizing.
+        // Always send an exposure event during a new frame to ensure forward progress if the frame callback already occurred.
         SDL_SendWindowEvent(window, SDL_EVENT_WINDOW_EXPOSED, 0, 0);
     }
 
@@ -1643,6 +1643,11 @@ static void decoration_frame_configure(struct libdecor_frame *frame,
         struct libdecor_state *state = libdecor_state_new(wind->current.logical_width, wind->current.logical_height);
         libdecor_frame_commit(frame, state, configuration);
         libdecor_state_free(state);
+
+        // Always send an exposure event during a new frame to ensure forward progress if the frame callback already occurred.
+        if (started_resize) {
+            SDL_SendWindowEvent(window, SDL_EVENT_WINDOW_EXPOSED, 0, 0);
+        }
     }
 
     if (wind->shell_surface_status == WAYLAND_SHELL_SURFACE_STATUS_WAITING_FOR_CONFIGURE) {