to avoid probelm with clang -O2 to outputting buggy code

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

My guideline for the rules is the PNG loader (which I consider
"canonical"). In the _load functions, x and y are required but
comp is optional; in the _info functions, all three are optional.

Fixes issue #411 (and other related, unreported issues).

We tried but it was nothing but trouble. New rule: with
GCC/Clang, if you're compiling with -msse2, you get always-on
SSE2 code, otherwise you don't get any. Trying to ship
anything with proper runtime dispatch requires both working
around certain bugs and some fiddling with build settings,
which runs contrary to the intent of a one-file library,
so bail on it entirely.

Fixes issue #280.
Fixes issue #410.

tweak license reference wording