Add https://github.com/jp7677/dxvk-nvapi as a submodule. dxvk-nvapi will
not be copied into Proton prefixes by default, but instead will be
controlled via the environment variable PROTON_ENABLE_NVAPI. This is
done to avoid any potential adverse effects of the nvapi DLL existing
in cases where an application may require a function that is not
implemented by dxvk-nvapi.
This new functionality can be enabled by setting the following environment
variable to a value of `1`:
`PROTON_ENABLE_NVAPI`
This functionality is needed in order to support DLSS within Proton.
Reviewed-by: Adam Moss <amoss@nvidia.com>
