Fix rendering of monochrome JPEGs at a scale of less than 1/6.
Robert Sprowson authored
The normalisation was wrong, leading to very dark images.
There was no code to deal with the 8bpp grey modes, leading to corruption.
This is the monochrome version of the same bug fixed for colour in 1.32.

Version 1.40. Tagged as 'SprExtend-1_40'
460ebff1