so that it does both U and V planes at the same time. This will have speed
advantages when using SSE2 (or higher) optimizations, since we can do both
the U and V rows together in a single xmm register.

This also renames filter16 to filter16y and filter8 to filter8uv so that it's
more obvious what each function is used for.

Originally committed as revision 24337 to svn://svn.ffmpeg.org/ffmpeg/trunk