nongnu: steam: Add mangohud support.
* nongnu/packages/steam-client.scm (steam-gameruntime-libs): Add mangohud and needed dependencies (imgui and spdlog). (make-container-wrapper): Expose /sys/class/drm, /sys/class/hwmon, and /sys/class/powercap (if it exists) for hardware monitoring. Add /usr/share/vulkan/implicit_layer.d, and link MangoHud.json files here, renaming the 32bit version to avoid a collision. Follow links in /share/vulkan/icd.d. Signed-off-by: Jelle Licht <email@example.com>