Soft particle-scene intersection, solid collider collision (#7)

### Soft particle-scene intersection
Billboarded particles produce hard edges when intersecting with scene
geometry, which is ugly. This PR introduces configurable soft particle
edges which fade in intersections with geometry, very useful for
smoke-like particle effects.
It also adds configuration options for rounded particle edge fading.

This feature requires the camera to have the depth prepass enabled (i.e.
the `DepthPrepass` component added). If it is not present, the behaviour
is as before.

### Solid collider collisions
Previously, all colliders were treated as hollow. This adds support for
solid colliders, and forces particles out of them.