Non-linear brushes

Currently in Spine, brushes can be used when setting weights or enabling soft selection in the [mesh tools](https://esotericsoftware.com/spine-mesh-tools), and the falloff size can be specified with `Feather`, but this falloff is currently linear. This can be useful in some cases, but if you are weighting or deforming a three-dimensional object, you may want to use a non-linear brush with a smooth falloff in the center and a higher rate of falloff at the outer edges. It would be useful to be able to switch brush modes between linear and non-linear brushes.

**Related forum thread:**
https://esotericsoftware.com/forum/d/28229-%E6%88%96%E8%AE%B8%E4%BD%A0%E4%BB%AC%E5%BA%94%E8%AF%A5%E7%9C%8B%E4%B8%80%E4%B8%8B%E8%BF%99%E4%B8%AA%E9%A1%B9%E7%9B%AE