ext/jpeg/gstjpegdec.*: Make mjpeg actually work and skip jpeg data parsing if we know that the input is packetized (i...

Original commit message from CVS:
* ext/jpeg/gstjpegdec.c: (gst_jpeg_dec_init),
(gst_jpeg_dec_setcaps), (gst_jpeg_dec_chain),
(gst_jpeg_dec_change_state):
* ext/jpeg/gstjpegdec.h:
Make mjpeg actually work and skip jpeg data parsing if we
know that the input is packetized (ie. each input buffer
is exactly one jpeg frame).