Separate cache files for each heightmap LOD

[WilliamLewww]

Closes #3199

To prevent gazebo from loading the wrong cache files, each heightmap LOD will have its own set of cache files.

The LOD is appended to the cache files' names (5 in the example):

[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00000000.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00010000.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00000001.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00010001.dat

[scpeters]

Closes #3199

To prevent gazebo from loading the wrong cache files, each heightmap LOD will have its own set of cache files.

The LOD is appended to the cache files' names (5 in the example):

[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00000000.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00010000.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00000001.dat
[Msg] Loading heightmap cache data: /root/.gazebo/paging/heightmap_bowl_2/gazebo_terrain_5_00010001.dat

thanks I'll go ahead and test this. It's possible that cache files are equivalent for any nonzero LOD values, in which case we could simplify this a bit, but I expect that this will fix the issue

[scpeters]

thanks I'll go ahead and test this. It's possible that cache files are equivalent for any nonzero LOD values, in which case we could simplify this a bit, but I expect that this will fix the issue

I've confirmed that this fixes the issue, but also that the cache files appear to be identical for any LOD > 0. I've suggested a code change inline that may reduce the number of cache files

[mogumbo]

Tested some big terrains using gzserver lod = 0 and gzclient lod = 3. Terrain looked good in both gzclient and cameras. Without this PR, the structure of the terrain had visible flaws.