Resizing transparent animated gifs keeps previous frames

[montanaflynn]

The original gif has a transparent background but when resized each previous frame is still visible.

I think this might be related to #80.

[montanaflynn]

Here is a fix for another animated gif bug with a similar problem (it keeps previous frames instead of changing them to transparent) on this repo:

https://github.com/jaschaephraim/pingpong/blob/master/img.go#L75-L85 https://github.com/jaschaephraim/pingpong/blob/master/util.go#L13-L25

Without transparent fix:

With transparent fix:

[willnorris]

Agreed that this seems related to #80. I hadn't spent a lot of time looking at all the edge cases relating to transparent gifs, so it doesn't surprise me terribly that there are some issues. Feel free to try sending a pull request to fix this, or I'll try to take a look when I've got some time (which likely won't be soon).