build a table that decodes both magnitude and value of small ACs in one go.

decode image to YCbCr format

this function is designed to support animated gifs, although stb_image doesn't support it

@OPTIMIZE: store non-zigzagged during the decode passes, and only de-zigzag when dequantizing

returns 1 if "a*b + add" has no negative terms/factors and doesn't overflow