Supporting global illumination for macOS/metal

[athenaz2]

Desired behavior

Currently, global illumination works on Ubuntu but not on macOS. It would be great for GI to work on macOS though.

Additional context

For gazebosim/gz-sim#2550, when the homebrew CI build runs a test with GI enabled, it crashes on macOS. Output below:

281: [ RUN      ] CameraSensorGlobalIlluminationTest.GlobalIlluminationEnabled
281: libc++abi: terminating with uncaught exception of type Ogre::FileNotFoundException: OGRE EXCEPTION(6:FileNotFoundException): Cannot locate resource CrossPlatformSettings_piece_all.metal in resource group General or any other group. in ResourceGroupManager::openResource at /tmp/ogre2.3-20230918-86352-14va4s/ogre-next-2.3.1/OgreMain/src/OgreResourceGroupManager.cpp (line 793)

I also noticed that the GI unit test also skips for Apple platform currently too.