Native bindings to the SOFT_direct_channels_remix extension.

This extension adds the ability to remix channels to the SOFT_direct_channels extension. By default, that extension drops input buffer channels that don't have a matching output channel. This really only works well for controlled setups where the input and output are known, but problems arise when these become variable. An option to remix input channels to the closest output channels will ensure the sound will still be audible with as little virtualization as possible.