Library doesn't get page-flip notifications

[emersion]

The library has no way to figure out whether a page-flip happened. This is necessary for tracking the priority of each layer (at least as we do now, a time-based tracking wouldn't require that).

I've deliberately ignored this for now and assumed a page-flip would happen on liftoff_output_apply. It probably doesn't matter that much, since we're interested in update frequency differences between layers, not the actual page-flip events. I'll leave this issue opened just in case this causes some unexpected issues.

[emersion]

As an alternative, a drmCrtcGetSequence call at the beginning of liftoff_output_apply would allow us to know how many frames have been displayed since the last libliftoff invocation.