The billow fractal is a slight modification of the `fbm()`

fractal. Before
adding the new layer onto the last, the new layer is modified by taking the
absolute value, multiplying by 2, and subtracting one. The result is that the
new value will not contain negative values and so will always add on top of
the old values. This function is intended to be used in conjunction with
`fracture()`

`billow(base, new, strength, ...)`

## Arguments

- base
The prior values to modify

- new
The new values to modify `base`

with

- strength
A value to modify `new`

with before applying it to `base`

- ...
ignored

## Examples

```
grid <- long_grid(seq(1, 10, length.out = 1000), seq(1, 10, length.out = 1000))
grid$simplex <- fracture(gen_simplex, billow, octaves = 8, x = grid$x,
y = grid$y)
plot(grid, simplex)
```