wlr_output: Add adaptive_sync_supported
This will let compositors know if changing adaptive_sync state has any chance of working. When false, then the current state is the only supported state, including if adaptive_sync is currently enabled as is the case for Wayland and X11 backends.
When true, changing state might succeed, but no guarantee is made. It just indicates that the backend does not already know it to be impossible.
Alternative to !4638 (closed)
Edited by Kirill Primak