support gif

some gif files are not strictly follow 89a.
DGifSlurp will return read head error or get record type error.
but the image still can display. so here should igron the error.

attached test gif file.
![img_1400044114652](https://cloud.githubusercontent.com/assets/70651/2967196/2b38c614-db29-11e3-989d-9f1f016f7128.gif)