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

returns 1 if the product of two ints fits in a signed short, 0 on overflow.

set up the kernels

read 16bit value and convert to 24bit RGB

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

decode image to YCbCr format

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