Added palette callback function

[welshcoder]

Hi,

I've been using this library for some time now, and it works very well.

However, I have an application that requires the palette to be manipulated every now and again. In order to achieve this, I've added a callback that is called during the draw callback function with the palette information. This callback allows the user to manipulate the palette before drawing it. The user can, for example, change the hue of the image without having to do so for each pixel individually.

I've had this working rather well in my application (some nice effects!). It does rely on a change in the AnimatedGIF library, as it needs information on whether it is global or local palette that's being passed. I've got a pull request on that library to do exactly this (bitbank2/AnimatedGIF#59 (comment)).

I hope you find this change useful.

Cheers,
welshcoder

[welshcoder]

The pull request in the AnimatedGif library that I referenced above has been merged into the master, so this pull request should work with the latest dev version of that library now.