* introducing new public interface in imgconvert.c

   * introducing new public interface in imgconvert.c
       + avcodec_get_pix_fmt
         converts textual representation of pixel format into
	 the actual id. Complements avcodec_get_pix_fmt_name.
       + avpicture_layout
         serializes given picture into a flat array.
	 Complements avpicture_fill.

   * adding a new option -pix_fmt to the ffmpeg, in order to control
     pixel format for the codecs that do support it, like rawvideo,
     for example.

   * reducing complexity of the rawvideo codec by splitting it in two
     and making it more reliable via hooking up to the avpicture_layout.
     Plus adding new FourCC as described here: http://www.fourcc.org

   * A tiny fix for avienc.c that makes avih and video strf consistent
     regarding codec FourCC.

Originally committed as revision 1842 to svn://svn.ffmpeg.org/ffmpeg/trunk