HiZ Buffer issues on Integrated GPUs (Intel, Apple Silicon)

[SRSaunders]

When the Hierarchical Depth Buffer is enabled (either for the old SSAO, or the new Filmic Post FX passes), I see problems on Integrated GPUs as follows:

1. On the master branch when running Vulkan on Linux + Intel iGPU: Game crashes after loading a level with a device lost error.
2. On the push constants branch when running Vulkan on M1 Apple Silicon GPU: Game crashes after loading a level with device out of memory followed by device lost errors. I suspect this one does not show up on the master branch since the code is far too slow over there (< 10 fps). To get a reassonable frame rate I have to test on the push constants branch.

If I set r_useHierarchicalDepthBuffers = 0 the problem goes away for both cases above. I have looked at the mipmapgen shader code and played with the parameters a bit, but I don't see anything obviously wrong, other than potential memory issues. I am wondering whether there is a subtle sync + resource release delay problem that is exposed on iGPUs that are slower and have more limited memory.

If you don't want to research this, I can easily patch it by adding the following code to DeviceManager_VK.cpp:

	if( !otherGPUs.empty() )
	{
--->		//FIXME: On integrated GPUs disable HiZ buffer to avoid device out of memory/lost errors
--->		r_useHierarchicalDepthBuffer.SetBool( false );

		m_VulkanPhysicalDevice = otherGPUs[0];
		return true;
	}

Please let me know what you want to do with this.