Fix Inaccurate Window Movement in Linux Framebuffer Backend

Windows would jump to unexpected positions when moving with the
mouse on the Linux framebuffer backend. This patch, tested on
Raspberry Pi 3B and virtual machines, ensures smoother and more
reliable window dragging behavior.

Author: huaxinliao

backend/fbdev.c

@@ -190,7 +190,8 @@ static bool twin_fbdev_update_damage(void *closure)
     twin_fbdev_t *tx = PRIV(closure);
     twin_screen_t *screen = SCREEN(closure);
 
-    if (!tx->vt_active && twin_screen_damaged(screen))
+    if (!tx->vt_active && (tx->fb_base == MAP_FAILED) &&
+        twin_screen_damaged(screen))
         twin_screen_update(screen);
 
     return true;