output-management: let unsent adaptive sync event imply lacking support
For clients it is useful to know if a particular head supports adaptive sync or not. For example in order to enable or disable UI elements.
A simple way to do this is to only sent the adaptive sync event when a head supports it.
This has been discussed before in !112 (merged) by @emersion and @ifreund, but it looks to me like the discussion didn't come to a conclusion. So here is a simple idea how to convey the information to clients. I think this is potentially simpler than adding another enum value but let me know what you think.