Black areas of GIFs can be encoded as transparent

[sjudd]

Sample GIF: http://media0.giphy.com/media/ToMjGpFXLkWjpPP58D6/200_d.gif.

This only happens with DiskCacheStrategy.RESULT, which implies the problem is in how the encoder is deciding what the transparent index for each frame should be.

[sjudd]

Assuming my understanding is correct, there will never be a perfect solution to this. We can't distinguish between black and transparent reliably. I can get around this case because 443 pixels out of 110307 overall are transparent (.4%), but a half dark grey and half transparent image will probably still break.