The init function is only compiled if MMX is enabled, the call
must use the same condition.

Originally committed as revision 22531 to svn://svn.ffmpeg.org/ffmpeg/trunk

Originally committed as revision 22523 to svn://svn.ffmpeg.org/ffmpeg/trunk

This moves the DWT functions from snow.c and dsputil.c to a file of
their own.  A new struct, DWTContext, holds the function pointers
previously part of DSPContext.

Originally committed as revision 22522 to svn://svn.ffmpeg.org/ffmpeg/trunk