jpeg reading: allow the user to specify scale_num/scale_denum in image_r...

libjpeg downscale images by a factor of n/m during image reading/decompression whenever scale_num and scale_denom are specified in the read options. This pull requests adds parameters to the gil jpeg settings structure to enable the user setting these values.

n/m are integer values, for m = 8, n may be in the range 1...16, the downscaling is very efficient as libjpeg directly accesses the respective coefficent(s) on the 8x8 pixel DCT.