diracdec: rewrite HQ slice decoding

Now coefficients are written to a buffer and are then dequantized by the
new SIMD dequantization functions. For the lower bands without enough
coefficients to fill a register (and hence they overwrite) the C version
of the dequantization function is used.

The buffer is per-thread and will be realloc'd if anything changes.
This prevents regressions and having to limit slice size.

Signed-off-by: Rostislav Pehlivanov <rpehlivanov@obe.tv>
