channelmix: Lower volume of rear speakers for 2.0 -> 4.0 upmixing

After implementing !1275 (merged) I realized that dialogs seem too low in volume compared to ambient sounds. I noticed that the PSD paper mentions that both the surround and center channels (in case of 5.1 upmixing) should be weighted by a factor of 1/sqrt(2).

This patch implements this weighting for 2.0 -> 4.0 upmixing. With this, the output sounds more natural and dialogs are clearer. The "surround feeling" however is a bit less.

It seems that the weighting by 1/sqrt(2) is not done for upmixing to 5.1 either, so it might be necessary to implement it there, too. I am however not sure if you want this to be done in the individual upmixing functions or when generating the mixing matrix?