Fix: inflation layer cost did not monotonically decrease with small inflation radii

[JustusBraun]

The InflationLayers cost function did exhibit some strange behavior with smaller inflation and inscribed radii.

This is the original cost layer:

This is the inflation layer with the original cost fading function:

As we can see the cost decreases first and then increases again at the end of the inflation radius.

I have plotted the original cost function with inscribed radius at 0.3m and inflation radius at 0.5m, the X-Axis is the distance from the Vertex being inflated. The plot assumes that the inscribed_value parameter is 1.0.

We can see that at the 0.3m the cost is about 0.1 and then increases to 1.0 at 0.5m.

I have changed the calculation of the alpha value to be linearly decreasing between the inscribed and the inflation radius. This then leads to the following cost function, again the inscribed radius is 0.3m, the inflation radius is 0.5m and the inscribed_value parameter is 1.0:

Here the cost starts with 1.0 at the inscribed radius (0.3m) and then decreases until it reaches 0 at the end of the inflation radius (0.5m).

The cost layer with the new function looks like this: