Fixed mpeg12 top field first flag value with field picture encoding.

Fixed mpeg12 top field first flag value with field picture encoding.

The relevent extract of the iso 13818-2 about the value of the syntaxical
element top_field_first of the Picture Coding Extension is:

"top_field_first -- The meaning of this element depends upon picture_structure,
progressive_sequence and repeat_first_field.
[...]
In a field picture top_field_first shall have the value '0', and the only field
output by the decoding process is the decoded field picture."

Originally committed as revision 24853 to svn://svn.ffmpeg.org/ffmpeg/trunk