pressure-vessel: Treat libXNVCtrl as part of graphics stack
MangoHUD uses this to get performance information for older NVIDIA hardware via the NV-CONTROL X extension.
This is not technically part of the NVIDIA driver (it's part of the nvidia-settings open-source module), and there's no guarantee that NVIDIA users will have it installed at all, but we're probably more likely to get a version compatible with the user's GPU and drivers if we prefer to take it from the same place as the actual driver.
Resolves: #82 (closed)
Edited by Simon McVittie