In shared mode, we previously tried to feed the full native format to
IsFormatSupported in the hopes that the "closest match" returned was
actually that.
Turns out, IsFormatSupported will always return the mix format if we
don't use the mix format's sample rate. This will also clobber our
choice of channel map with the mix format channel map even if our
desired channel map is supported due to surround emulation.
The solution is to not bother trying to use anything other than the mix
format sample rate. While we're at it, we might as well use the mix
format PCM sample format (always float32) since this conversion will
happen anyway and may avoid unecessary dithering to intermediate
integer formats if we are already resampling or channel mixing.
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