Request: add bitmap cache, to avoid re-decoding when already done before

[AndroidDeveloperLB]

Whoever uses the library could set how much memory the bitmap cache of the GIF decoder should be used. If it's 0, it should be as it is today, meaning no caching. If it's <1, it should take as much as the library thinks that it's possible to have. If it's >0, this is the number of bytes in total, of all bitmaps, that the library should be allowed to have.