Qt 5 to-do in QPixmap code

// Legacy, single instance hooks: ### Qt 5: remove
typedef void (*_qt_pixmap_cleanup_hook)(int);
typedef void (*_qt_pixmap_cleanup_hook_64)(qint64);
typedef void (*_qt_image_cleanup_hook)(int);
Q_GUI_EXPORT _qt_pixmap_cleanup_hook qt_pixmap_cleanup_hook = 0;
Q_GUI_EXPORT _qt_pixmap_cleanup_hook_64 qt_pixmap_cleanup_hook_64 = 0;
Q_GUI_EXPORT _qt_image_cleanup_hook qt_image_cleanup_hook = 0;
Q_GUI_EXPORT _qt_image_cleanup_hook_64 qt_image_cleanup_hook_64 = 0;