Add underwater sfx for SDL backend

Underwater sound effect for SDL backend.

New console variables (affects both backends: SDL and OpenAL):
- __s_underwater__  
Enables (non-zero value) or disables (zero value) the effect.  
Default value: 1.0
- __s_underwater_gain_hf__  
Controls a gain of high frequencies.  
The higher the value - the weaker the effect.  
Default value: 0.25  
Range of values: [0.0; 1.0]  

Changes take effect immediately - you do not need to restart sound system with __snd_restart__.