Audio: Implement stricter panning

Reworked a panning a bit according to https://github.com/teeworlds/teeworlds/issues/22
The max distance now depends on the screensize and implemented a constant pan law (volume of left+right should stay independent of the source position)
Also removed the 'pan' variable of the channel as there is no use to it. 
>[13:55:10] matricks: heinrich5991: dunno actually what the panning parameter should do, don't know if I had a specific plan for it

thanks to matricks and heinrich5991 for help (: