#29 Underwater sfx

[Boris I. Bendovsky]

The patch adds an underwater sfx based on OpenAL Soft implementation of low-pass filter.
Two new cvars controls the effect.

snd_underwater_sfx
Enables (non-zero value) or disables (zero value) the effect.
Default value: 0

snd_underwater_sfx_gain_hf
Controls a weakness of the effect. The higher a value the weaker the effect.
Valid range of values: [0.0, 1.0]
Default value: 0.25