fix incorrectly-named assert, attempt to fix (char) cast warning (tricky since char's signedness is unknown)

this variable was causing a warning while compiling with gcc.

tweak docs

Also ran a bunch of test cases to make sure the IDCT and H2V2
resamplers were correct.

Conflicts:
	stb_image.h

Put progressive JPEG AC decode logic back the way I wrote it originally (I changed it to match jpgd when I was trying to figure out why it didn't work);
add STBI__ prefixes to internal SCAN_ enum;
strip unused function arguments for progressive funcs;
tweak release notes;
forget to git commit frequently so these would all be in their own commits;