compress: Apply damping later.

This changes the algorithm of the compress filter to apply the right
shift defined by the --damp argument *after* the peak value of the
current block has been computed. This makes damping orthogonal to
the dynamic volume adjustments.

However, we increase the downshift so that the initial value of the
gain multiplier results in a right shift by one. Hence the expected
peak value is 16384, so make this the default target gain.