M/S stereo sucks: use L/M/S stereo instead!

The previous version of the filter sounded decent, but by accident. There
were serious bugs in the filter designer, and fixing those revealed it was
probably impossible to make this filter work the way I wanted in M/S stereo.
Thankfully, there's more than one lossless way to decompose stereo!
Hopefully this one works out better.

This commit also ditches the iterative filter designer in favor of a more
analytical approach. Why guess and check when you can compute your way to a
correct result?