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

after we see SOS

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* @brief Allocates property fields + keys. * @param numProperties Number of requested properties. */

Append a string to the string */