Fix Inaccurate Window Movement in Linux Framebuffer Backend

Windows would jump to unexpected positions when moved with the
mouse on the Linux framebuffer backend. By suppressing the screen
update behavior of twin_fbdev_update_damage, this patch ensures
more robust window dragging on Raspberry Pi 3B and virtual machines.

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;