Fix memory region in framebuffer and pan display

The width of a screen may be different to the virtual resolution.

Author: a1091150

backend/fbdev.c

@@ -56,7 +56,7 @@ static void _twin_fbdev_put_span(twin_coord_t left,
         return;
 
     twin_coord_t width = right - left;
-    off_t off = top * screen->width + left;
+    off_t off = top * tx->fb_var.xres_virtual + left;
     uint32_t *dest =
         (uint32_t *) ((uintptr_t) tx->fb_base + (off * sizeof(uint32_t)));
     memcpy(dest, pixels, width * sizeof(uint32_t));
@@ -82,9 +82,13 @@ static void twin_fbdev_damage(twin_screen_t *screen, twin_fbdev_t *tx)
 static bool twin_fbdev_work(void *closure)
 {
     twin_screen_t *screen = SCREEN(closure);
+    twin_fbdev_t *tx = PRIV(closure);
 
-    if (twin_screen_damaged(screen))
+    if (twin_screen_damaged(screen)) {
         twin_screen_update(screen);
+        ioctl(tx->fb_fd, FBIOPAN_DISPLAY, &tx->fb_var);
+    }
+
     return true;
 }