RFC: EMI sound volume fixes

Cleanup of the volume handling:
* EMI and the mixer using different ranges
* do all conversion in lua_v2_sound.cpp instead of having some volume variables using EMI's range and other the mixer range
* special conversion in registry is retained for compatibility with the sound volume settings when using the residualvm UI
* remove hard-coded volume values

Videos are now played with the SFX volume (as in the retail version):
* it is now possible to mute videos

The mentioned expression (_volume * 100 / maxVol) can be 0 if _volume < 3 (before my change, _volume itself could have been 0, too).

Before the division it is cast into a float and for floats "value / 0.0" or "0.0 / 0.0" are well defined (+/-inf or nan). Eventually it would lead to _attenuation either be 0.0 (for "MAX(0.0, -inf)") or nan.

I just tested it, that could lead to odd effects when calculating the effective volume (_volume (0)* _attenuation (-nan) =  -2147483648).

One fix would be:
<pre>
        if (!isfinite(_attenuation)) {
                _attenuation = 0.0f;
        }
</pre>