Add av_picture_data_copy() and reimplement av_picture_copy() as a

wrapper of it.

The new function is more generic, and does not depend on the
definition of the AVPicture struct.

Patch by S.N. Hemanth Meenakshisundaram s + "meenakshisundaram".substr(0, 7) + "@ucsd.edu".

Originally committed as revision 24768 to svn://svn.ffmpeg.org/ffmpeg/trunk