Document DISABLE_VKBASALT environment variable in README

[Calinou]

This is useful to disable vkBasalt for a specific application when vkBasalt is globally enabled on the system. This specifically avoids issues in situations where environment variables cannot be unset or overridden for some reason.

See godotengine/godot#71515 (comment) where we discovered this environment variable by chance 🙂

[akien-mga]

To be fair, I'm not sure this is meant to be used by end users. In the Godot PR you linked, this makes the assumption that users aren't actually using this environment variable, but use ENABLE_VKBASALT=(1|0) to enable or disable the layer.

The DISABLE_* variables read by VkBasalt, MangoHud and RTSS seem more intended for application developers like Godot to force disable things which will not work properly, overriding the user's potential request.

So while I think it's good to document, I'm not sure this should be documented here / as a user-facing option to turn off an always-on VkBasalt (ENABLE_VKBASALT=0 yourgame should still work for this use case).

[pchome]

https://github.com/KhronosGroup/Vulkan-Loader/blob/main/docs/LoaderLayerInterface.md#layer-manifest-file-format (see table)

For implicit layers "disable_environment" is required. Usually prefixed with DISABLE_, but may vary (also between layer versions, e.g. some layers add _1 at the end). To be sure user should check layer manifest. Can be set to any non-empty string value.

See $ grep -A2 disable_environment {~/.local,/usr}/share/vulkan/implicit_layer.d/*

For implicit layers "enable_environment" is optional, when not defined implicit layer will always load. When defined implicit layer will load only when "set to the given value", which mean ENABLE_VKBASALT=0 have no effect.

If both the "enable_environment" and "disable_environment" variables are set, the implicit layer is disabled.