Move H264 dsputil functions into their own struct
This moves the H264-specific functions from DSPContext to the new
H264DSPContext. The code is made conditional on CONFIG_H264DSP
which is set by the codecs requiring it.
The qpel and chroma MC functions are not moved as these are used by
Originally committed as revision 22565 to svn://svn.ffmpeg.org/ffmpeg/trunk