Difficult to orbit around heightmaps

## Environment
* OS Version: Ubuntu Focal
* Source or binary build? Source, `main` or `ign-rendering6`

## Description
* Expected behavior: Using the mouse to orbit around a heightmap should be as easy as orbiting around other meshes.
* Actual behavior: It looks like the ray doesn't intersect the heightmap and I'm orbiting around an arbitrary point in space.

See how it moves much more when orbiting the sphere than on the heightmap:


This becomes more problematic the further you are. Here's an example with a large heightmap. Orbiting the grid is much easier:


## Steps to reproduce

1. `ign gazebo heightmap.sdf`
2. Orbit around the heightmap and the other models

## Additional context

I remember we had the same issue on Gazebo classic a while back:

* Issue: https://github.com/osrf/gazebo/issues/2049
* PR diff: https://github.com/osrf/gazebo/compare/482a93b0877be937651ff85413ece0ca7718b23a..9a92f437ef54261bbdbd09243a8a7b7fdb0fa95b

Start by reproducing the issue with `ign gazebo heightmap.sdf`, comparing orbiting the heightmap with the other models and the grid. Review the linked Gazebo classic issue and PR diff for prior context. Done means mouse orbiting around heightmaps is as easy and stable as orbiting around other meshes, including at larger distances.

Written by the indexing model from the issue text.