`compositor::Compositor` should allow recompositing request without stop/start

This is the latest example, but there are others in the surrounding code:
```c++
else if (configuration_changes_require_recompositing(*existing_configuration, *conf))
{
// Workaround for #2807: we could do better if Compositor had a `schedule_compositing(damage)` method
ApplyNowAndRevertOnScopeExit comp{
[this] { compositor->stop(); },
[this] { compositor->start(); }};
}
```

Start by tracing the surrounding configuration_changes_require_recompositing path and the compositor->stop()/start() workaround shown in the issue. Read the compositor::Compositor entry points and determine how a schedule_compositing(damage) request could replace the restart; done means recompositing can be requested without stopping and starting the compositor.

Written by the indexing model from the issue text.