Fix dec artifacts w/cropped+smoothed prog DC scans This commit modifies decompress_smooth_data(), adding missing MCU column offsets to the prev_block_row and next_block_row indices that are used for block rows other than the first and last. Effectively, this eliminates unexpected visual artifacts when using jpeg_crop_scanline() along with interblock smoothing while decompressing the DC scan of a progressive JPEG image. Based on: https://github.com/mo271/libjpeg-turbo/commit/0227d4fb484e6baf1565163211ee64e52e7b96bd Fixes #456 Closes #457