output: fail commits if adaptive sync cannot be enabled

This is just !2976 (closed) rebased onto master with conflicts resolved.

Implementing this is a prerequisite for implementing wlr-protocols!112 (merged) in wlroots.

From the commit message:

Previously, adaptive sync was just a hint and wouldn't make any atomic commit fail if the backend didn't support it. The main reason is wlr_output_test wasn't supported at the time.

Now that we have a way for compositors to test whether a change can work, let's remove the exception for adaptive sync and convert it to a regular output state field.