EDF Ambient Sounds!

In preparation of the addition of sound sequences, and mostly due to Mordeth's recent feature request, I have added ambience definitions to EDF.

There's a brief example here:

http://doomworld.com/eternity/engine/tutorials/ambient_test.edf

As in zdoom, the first 64 ambience indices from 1 to 64 can be placed into the map using mapthings 14001 through 14064. Thing 14065 (EEAmbience in EDF) accepts an ambience index number in args[0], and requires Hexen map format or ExtraData to be used.

Several new sound engine features are utilized by this and will also be needed for sound sequences. These include the ability to scale the volume of individual sound channels and to specify custom attenuation behavior on a per-channel basis as well. Eternity supports 4 attenuation types, normal (uses the data defined in EDF), idle (uses DOOM's old defaults all the time), static (fades out fast), and none (plays everywhere at the channel volume level).

As a side effect, these features together enable global sounds that don't necessarily have to play at full-volume :)

Thanks!