stb_image: JPEG resampler func for NEON
stb_image: NEON integer IDCT (not yet tested!)
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;
Merge branch 'fastjpeg' of https://github.com/nothings/stb into fastjpeg

Conflicts:
	stb_image.h
stb_image: NEON YCbCr->RGB kernel.

Also ran a bunch of test cases to make sure the IDCT and H2V2
resamplers were correct.
Merge branch 'fastjpeg' into working
cleanup merge of ARM NEON support
tweak docs
add #defines to allow disabling decoders for unwanted formats