idle-inhibit: allow inhibiting all outputs

I maintain caffeine-ng, a tiny tray application where a user can manually toggle inhibiting screen powering off.

I'd like to inhibit screens being turned off for all outputs, but the current protocol requires that I provide a surface and will only inhibit idle for that one. I can hack around this with layer shell and create a surface on each output, but this is a terrible hack.

I'd like to be able to inhibit idle for all outputs -- perhaps with a new request, perhaps with just surface=null. Any thoughts on this?