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
non-h264 code.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22565 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b