Further cleanup of the AltiVec forward DCT code:

Further cleanup of the AltiVec forward DCT code:
-- Use macros to represent the fast FDCT constants, to facilitate comparing the AltiVec implementation of the algorithm with the SSE2 implementation.
-- Rename slow FDCT constants for consistency.
-- Use vec_sra() in all cases in the slow FDCT code.  The SSE2 implementation uses psraw, which is an arithmetic shift, so we need to do likewise with AltiVec.  Using vec_sr() hasn't caused any problems yet, but it is conceivable that it might cause different behavior in certain corner cases.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1444 632fc199-4ca6-4c93-a231-07263d6284db