This stores the caps array in 32nd bits/sample instead of 1/2 bits
scaled by LM and the channel count, which is slightly less
less accurate for the last two bands, and much more accurate for
all the other bands.
A constant offset is subtracted to allow it to represent values
larger than 255 in 8 bits (the range of unoffset values is
77...304).
In addition, this replaces the last modeline in the allocation table
with the caps array, allowing the initial interpolation to
allocate 8 bits/sample or more, which was otherwise impossible.
Previously, we would only split a band if it was allocated more than
32 bits.
However, the N=4 codebook can only produce about 22.5 bits, and two
N=2 bands combined can only produce 26 bits, including 8 bits for
qtheta, so if we wait until we allocate 32, we're guaranteed to fall
short.
Several of the larger bands come pretty far from filling 32 bits as
well, though their split versions will.
Greg Maxwell also suggested adding an offset to the threshold to
account for the inefficiency of using qtheta compared to another
VQ dimension.
This patch uses 1 bit as a placeholder, as it's a clear
improvement, but we may adjust this later after collecting data on
more possibilities over more files.
Use measured cross-entropy to estimate the real cost of coding
qtheta given the allocated qb parameter, instead of the entropy of
the PDF.
This is generally much lower, and reduces waste at high rates.
This patch also removes some intermediate rounding from this
computation.
