vulkan/overlay: add cpu-load graphs

Hi,

This MR basically ports the cpu-load-query from the gallium_hud to the vulkan-overlay. The new graphs can be added to the overlay by setting the new stat flags 'cpu-load' for an average, or 'cpu-load-per-core' for one graph per cpu. I decided against copying the windows implementation from the gallium_hud because i currently have no way to test it. There is also one indentation fix in there which i spotted along the way. Feel free to demand major changes, as i'm not really sure about the approach, thanks!.