===== Band envelope processing =====
The band envelope is given as a pair of X/Y values to represent a step function. The X values are quantized down to 32 instead 128 possible values. The Y values are exponents for the base 2, the range is -6 to 9 (so 1/64..512). The envelope will be expanded with geometric interpolation (resulting in 3 interpolated samples) on the step. The left half of the IMDCT result will be scaled by the previous envelope multiplied by the first envelope Y value of the right half.

IMDCT overlapping will be performed immediately after envelope processing.