Module baal::effect::persistent

this module allow to play persistent sound effects

volume = global_volume * effect_volume * sum(distance(position,listener_position))

but once a sound effect is played at a volume it doesn't change its volume anymore

this can lead to weird effects for not so short sound effects and with moving source

also if its volume is zero then the sound is not played at all

Functions

add_position	add a new source of the effect
add_positions	add a vec of new sources of the effect
add_positions_for_all	add a vec of new sources of the effects
clear_positions	remove all sources of the effect
clear_positions_for_all	remove all sources of all effects
update_volume	update the volume of effect computed from sources position and listener position at the moment of this call
update_volume_for_all	update the volume of all effect