improve image test
improve image test

- list failed tests like trace.c
- use imagediff (taken from cairo) to compare images
- dump diff image in dump mode

freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec