graphics: Load ICDs and layers to match Vulkan-Loader >= 1.2.198
It was previously non-compliant with the XDG basedir spec, but this has now been fixed. Catch up with how it now works:
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XDG_CONFIG_HOME is now searched (previously it was not)
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XDG_DATA_HOME and ~/.local/share are now higher-precedence than XDG_DATA_DIRS
The fact that we search both XDG_DATA_HOME and ~/.local/share, even if set, actually turns out to be a mistake (I misread the reference Vulkan-Loader code), but it makes pressure-vessel accidentally work around a Steam bug, so keep it for now. We deduplicate the search paths automatically, so in the common case where XDG_DATA_HOME is unset or points to ~/.local/share, it's harmless to look at it twice.
Part of steamrt/tasks#68