Fix duplicate vertex program name crash

[Ace314159]

I'm on Windows 10, using Ogre 1.12.9.

On launch, Gazebo crashes due to the following error, which I found in the logs:

[D:\vcpkg\buildtrees\gazebo11\src\gazebo\server_main.cc:57] Ogre Error:Ogre::ItemIdentityException::ItemIdentityException: HighLevelGpuProgram with the name shadow_caster_vp_glsl already exists. in ResourceManager::add at D:\vcpkg\buildtrees\ogre\src\OgreMain\src\OgreResourceManager.cpp (line 149)

I was able to determine that it started occurring after #3051. However, the file causing the issue was changed because it couldn't find the vertex program with that name, brought up here.

In order to fix both issues, I renamed the vertex program to be unique. This fixes my crash, but I'm not familiar with Ogre and how it handles shaders, so I'm not sure if it'll fix the original issue as well. I'm assuming if the CI looks good, it means it worked? If there's a better way to address this issue, I'm happy to do that as well.

[WilliamLewww]

This should work fine because the program still directs to the correct vertex program source:
https://github.com/osrf/gazebo/blob/ea2e1075b1132d509e5b368c7751f75a937f7103/media/materials/scripts/shadow_caster_ignore_heightmap.program#L3