The creator of MagicaVoxel just released a new free interactive path renderer for heightmaps called AerioLOD. It is a very early version, labeled 0.0.0 and available for 32 and 64bit Windows machines.
The very brief description from the homepage:
An interactive path tracing renderer for height maps.
- support rendering height maps of size up to 16384^2.
- support importing and exporting 8-bit and 16-bit png images.
Current release notes:
0.0.0 - 10/19/2019
You can check out AerioLOD in action in the video below.