Unit test coverage: Heightmaps on all cameras

After merging PR #545, codeconv reports lower coverage because the heightmap is not being tested in any sensor:

- [x] `Ogre2MaterialSwitcher` (e.g. VisualAt / SelectionBuffer)
- [x] `Ogre2LaserRetroMaterialSwitcher`
- [x] `Ogre2SegmentationMaterialSwitcher`
- [ ] `Ogre2ThermalCameraMaterialSwitcher`
- [ ] Heightmap is a background object (i.e. has no thermal value defined, uses ambient/color instead)
- [ ] Heightmap has an explicit thermal value specified
- [ ] Heightmap does not support heat map texture setting. Unit test should test we properly abort/complain if user requests it.

These tests can only be implemented in Garden, not Fortress (as Heightmap doesn't work with those sensors in Fortress, we just disable the terrain as a workaround).

A unit test (or extending existing tests) to cover the heightmap with each sensor should be written.

This is very low priority for me and anyone can implement it.