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