Voxel depth testing doesn't work with multi-frustum rendering

Voxels are always rendered in the first frustum and only have access to the depth texture in that frustum, which means that depth testing doesn't work most of the time when doing multi-frustum rendering, e.g. when `viewer.scene.logarithmicDepthBuffer = false;`.

Start by reproducing voxel rendering with viewer.scene.logarithmicDepthBuffer = false and observe depth testing during multi-frustum rendering. Trace how voxels are assigned to the first frustum and how that frustum's depth texture is accessed; done means voxel depth testing works across multi-frustum rendering rather than only in the first frustum.

Written by the indexing model from the issue text.