avcodec: add avdct

This provides a public sustainable API/ABI for DCT functions.
Only externally used dct functions are included.
The structure is extensible without ABI issues compared to the
existing dct contexts.

See Mailing list and IRC log of 2014-07-26/27

Reviewed-by: ubitux
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_spp: use AVDCT

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>