ext/jpeg/gstjpegdec.c: Fix decoding of pictures with certain uneven or unaligned...

ext/jpeg/gstjpegdec.c: Fix decoding of pictures with certain uneven or unaligned widths where jpeglib needs more hori...

Original commit message from CVS:
* ext/jpeg/gstjpegdec.c: (gst_jpeg_dec_decode_indirect),
(gst_jpeg_dec_decode_direct), (gst_jpeg_dec_chain):
Fix decoding of pictures with certain uneven or unaligned
widths where jpeglib needs more horizontal padding than our
I420 buffers provide, resulting in blocky artifacts at the
left side of the picture (#164176).
Also make use of our shiny new GST_ROUND_N() macros.