Function dbap::blurred_distance_2

pub fn blurred_distance_2<S>(source: [S; 2], speaker: [S; 2], blur: S) -> S where
    S: Scalar, 

The same as a regular distance function but applies a subtle blur amount.

From the paper: "In 2D space, blur can be understood as a vertical displacement between source and speakers. The larger ` gets, the less the source will be able to gravitate towards one speaker only."

A non-zero blur will ensure that the distance is greater than 0.0 and that we never divide by 0.0.