Implement PixelRegion option for imread(). * imread.m: document new option. * private/__imread__.m: parse new option and set defaults. * __magick_read__.cc (calculate region): new option to calculate the region to be read, shifts in memory required, and output image size. (read_indexed_images, read_images): implement reading of only specific regions of an image. (__magick_read__): get octave_scalar_map at start for simplicity.