Mozilla Bug 893307

Copy the left channel to the right channel when playing mono on
surround sound system using WASAPI.
Mozilla Bug Bug 893307

Compute the number of bytes per frame before upmix in a correct way when using
surround speakers in the WASAPI backend.
Mozilla Bug 900380

Don't assume GetMixFormat gives us a WAVEFORMATEXTENSIBLE, even if the docs say
so.
Merge pull request #32 from padenot/master

Mozilla bugs 893307 & 900380