#4 Stereo channel panning

[Rene Schickbauer]

Output of effects should include the ability to pan left/right.

Description: To implement "follow the object sound", it is required to implement output filters sound module of the engine

Usage-Example: A fire object makes a burning sound. To hear direction and distance relative to the screen center, the level script (or possible the engine itself) could change the left and right volume of the sound effect.

Software requirements:
* Add filters to all effect channels.
* Adapt/Enhance scripting functions
* Add internal "screen position" and "loop" properties to sound effects or alternatively use sound effects and virtual fgObjects

[Rene Schickbauer]

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Will implement this in the new OO-style lua bindings.