Ellipsoid Primitive clipping

When the ellipsoid primitive was positioned on the intersection of two frustums and close to the  surface, the terrain in the closest frustum would overwrite part of the ellipsoid that was rendered in the farther frustum.

Clamping the depth in the vertex shader causes artifacts for translucent objects because some fragments can be alpha blended twice.

Rendering the ellipsoid in the closest frustum only and clamping the depth fixes this issue as well as the near plane clipping. Also, the issue where you can see the multi-frustum boundaries on translucent ellipsoids disappears because it is only ever drawn in one frustum.

This resolves part of #390.