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

int raw_data[4] to 0, or the compiler bugs

more of "allocate large structures on the stack", this time in the forgotten stbi__jpeg_test
-> avoids the infamous _chkstk() when working with CRT