vp3: move idct and loop filter pointers to new vp3dsp context

This moves all VP3-specific function pointers from dsputil to a
new vp3dsp context.  There is no reason to ever use the VP3 IDCT
where an MPEG2 IDCT is expected or vice versa.

Signed-off-by: Mans Rullgard <mans@mansr.com>