JCOEF is a short, it can be changed to an int for a speed increase.
JCOEFPTR is "JCOEF FAR *" (currently a short *).
JSAMPLE is unsigned char
JSAMPROW is "JSAMPLE FAR *" (pointer to one image row of pixel samples)
inptr is is JCOEFPTR, but it would probably be faster as an int ptr.
quantptr is an int32* or a uint32* (probably an int32* since imul is used)
DEQUANTIZE appears to do nothing more than multiply.