Conflicts:
	stb_image_write.h

to avoid probelm with clang -O2 to outputting buggy code

Only clear to background color if index is non-zero.
Fixed a the disposal test gif I was using - now renders properly (gif has no transparent set, but all renderers still considered it transparent.  Spec says 0 should be ignored if 0, but was confusing by saying it only in the context of the pal not existing.. but seems to be the case always.

Per the contributor doc - added my name.   Noticed urraka also did some work here, so hopefully didn't step on any toes.

- Fix an issue where the spec of the gif for restore to previous uses code 3, not 4.
- To get results that worked - made an assumption that "clear to background" meant "revert back to what was there before I drew", where mode 1 would revert back to the previous frame [slightly different].  If I clear to background color instead, I ended up with large opaque squares in gifs that changes their transparent colour each frame.
- Background color is supposed to be used only for pixels not rendered by the image, so took that to mean it only really affected the previous frame, or potentially any frame that used full disposal.  Since background color is allowed to be unspecified this is what lead me to believe I shouldn't use it for disposal.
- Oh, also upped the codes table to 8192 as 4096 ended up being too small for a few of my test cases.

Full disclaimer - I only read through the GIF format for this contribution, so competly could be misinterpreting the spec - but this gave me reuslts that matched Chrome.

- Allow loading a gif as multiple frames into a single buffer.  Each frame is a full image seperated by a (w * h * comp) stride.
- Optionally, can pass in a pointer to a int, which will be filled with an array layers long contain ms for each frame.
- Fix gif's not loading the initial transparent background
- I believe also fix disposal rules for subsequent frames (though being somewhat inefficient with memory to do so)
- Add a flip_vertical that takes into account slices as well.

Compiled using VS2017, but nothing else as I'm not really setup for it.  Apologies.

This reverts commit 9dfa8c7f.