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.

So we report channels_in_file correctly.

Fixes #329.

This incorporates #462, but also factors everything into one
function that is shared between 8-bit integer, 16-bit integer, and
float pixels (vertical flip operates on rows of bytes and doesn't
really care), and finally always uses a 2k on-stack buffer without
dynamic memory allocation, doing multiple memcpys per row if
necessary. Not only does this remove an out-of-memory failure mode,
it is also preferable for large images, since it's more
L1-cache-firendly this way.

Fixes #462.

This is the same method as in pull request #455, but using integer
arithmetic instead of converting to float.

Fixes #455.

req_comp is now desired_channels and *comp is *channels_in_file.

Fixes issue #466.

We used to require exact match between img_len and raw_len for
non-interlaced PNGs, but the PNG in issue #276 has extra bytes
(all zeros) at the end of the compressed DEFLATE stream.

The PNG spec doesn't have anything to say about it (that I
can tell), and if libpng accepts this, who are we to judge.

Fixes issue #276.

Function `stbi__sse2_available` takes no argument,
we should be explicit by passing `void` as argument.
It will remove warnings from 'some' compilers.

so you don't get the automatic info you normally do