Cache vehicle positions across draw blocks in viewport map mode

src/screenshot.cpp

@@ -672,6 +672,8 @@ static void LargeWorldCallback(void *userdata, void *buf, uint y, uint pitch, ui
 	/* Switch back to rendering to the screen */
 	_screen = old_screen;
 	_screen_disable_anim = old_disable_anim;
+
+	ClearViewPortCache(vp);
 }
 
 /**