mpeg SVCD compatibility, SCR fixes, standard compliance

- fixed VBR+constrained bitstream header flags for non-VCD
- more sane (and SVCD compatible) value for video stream->max_buffer_size
- always write at least one PES header stuffing byte for MPEG-2 to
prevent accidental start code generation
- do not write more than 16 stuffing bytes in a PES header (not
allowed). Use padding packets instead.
- include a PES extension in the first MPEG-2 packet
- fill the first pack of SVCD files with padding
- "sanity hack" that prevents the SCR from overtaking the PTS for non-VCD
- fixed VCD PTS values to correspond to the SCR
- always include DTS in the first SVCD packet (fixes lots of
compatibility problems with DVD players)

patch by (Hauke Duden <H.NS.Duden at gmx dot net>)

Originally committed as revision 3078 to svn://svn.ffmpeg.org/ffmpeg/trunk

tests/libav.regression.ref

@@ -7,7 +7,7 @@ b166e89a9ec8c707573329d883c1b6f9 *./data/b-libav.asf
 ./data/b-libav.asf CRC=525fdb22
 be8eb1b5705c8105e4727258e448cb24 *./data/b-libav.rm
  356950 ./data/b-libav.rm
-072645ee8b36e432227cf9dff21561f0 *./data/b-libav.mpg
+e826aa1637ff15144ab484c1efca7fe7 *./data/b-libav.mpg
  382976 ./data/b-libav.mpg
 ./data/b-libav.mpg CRC=eda0e29e
 01a4130e776b8955fa99e477113e94fd *./data/b-libav.swf