BGameSound: cleanup and several fixes

* Use a template to avoid repeating the panning code for each type
* Rearrange the initialization of BSimpleGameSound so the data is only
copied 0 or 1 times (it was copied 2 or 3 times in some cases) between
the passed buffer and the final one,
* Don't execute the panning code on streams with only 1 channel as this
makes no sense and it will read and write outside the buffers
* Abort early in GameSoundBuffer::Play if fIsPlaying is false.

Fix the crashes in Worms Armageddon. Doesn't fix the sounds being mostly
white noise.