idctdsp: Add global function pointers for {add|put}_pixels_clamped functions

These function pointers already existed in the ARM code. Adding them globally
allows calls to the function pointers to access arch-optimized versions of the
functions transparently.
xvid: Add C IDCT

Thanks to Pascal Massimino and Michael Militzer for relicensing as LGPL.

Signed-off-by: Diego Biurrun <diego@biurrun.de>