Unroll loop in h264_idct_add8_sse2(). This means we can inline scan8[] in the
code directly also and remove loop setup. 20% faster in function, 0.8% overall.

See "[PATCH] unroll loop in h264_idct_add8_sse2()" thread on ML.

Originally committed as revision 25171 to svn://svn.ffmpeg.org/ffmpeg/trunk
