Added ability to manually get / set _transform.pan

I use this as a cheaper alternative to "proximity" which only seems worth using for looped sounds. I set the volume and pan of the sound based on distance from center of camera and then just play the sound, no overheads after the initial calculations.
Fixed getter.
Variable name change from _panning to _proximityPan to avoid confusion
Fixed getter.
Inlined get_pan / set_pan
Merge pull request #842 from PaulGene/patch-1

Added ability to manually get / set _transform.pan