C optimizations for DC prediction.

C optimizations for DC prediction.
These are relatively minor on modern platforms (1.6-2.7% in the decoder on
 x86-64, less than 1% on the encoder), but can have a much larger impact on
 older platforms/underpowered CPUs with poor branch prediction.
Modified version of a patch by Simon Hosie.

svn path=/branches/theora-thusnelda/; revision=16332