Use OpenAL's linear attenuation model

We should use the inverse distance clamped model (the default), but we first
need to handle muting sounds that are beyond their max distance. Linear
attenuation doesn't give a proper rolloff, but it makes the sounds silent at
max distance.

apps/openmw/mwsound/openal_output.cpp

@@ -303,6 +303,8 @@ bool OpenAL_Output::Initialize(const std::string &devname)
         Device = 0;
         return false;
     }
+    alDistanceModel(AL_LINEAR_DISTANCE_CLAMPED);
+    throwALerror();
 
     return true;
 }