Check the absolute gain value for silence

Future B-Format support will be using negative gains, which still need to be
applied.
Add preliminary AL_EXT_BFORMAT support

Currently missing the AL_ORIENTATION source property. Gain stepping also does
not work.
Rename the source's Orientation to Direction
Support B-Format source rotation with AL_ORIENTATION
Add AL_EXT_MULAW_BFORMAT to alext.h
Support AL_EXT_MULAW_BFORMAT