Openal t2d fixes

This implements a few missing OpenAL functions required by Torque2D. Namely: alIsBuffer, alGetSourcefv, alGetListenerf, alGetListenerfv, alGetListeneri, alListenerf, alEnable and alDisable.

It also adds a basic master gain property to the OpenAL listener. Judging by the OpenAL-soft code, a 100% correct solution might need to be a bit more elaborate, but then again I cannot find an exact specification on how this value should be interpreted.

Addresses issue #1923.