Since IDCT transforming 32-bit input to 8-bit output is unusual and unpractical
for most codecs, move Bink IDCT into separate context. Get rid of an additional
permutation table while at it since SIMD support for Bink IDCT is unlikely to
be implemented in foreseeable future.
Quantisation tables also have to change type to signed for proper
dequantisation of DCT coefficients.

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