vblankmanager: Treat wakeup time as scheduled wakeup point for timerf…

…d path

Because we are using timerfd, ulWakeupTime should actually be the target point of the timerfd when using that, not the the current time from OnPollIn, so we can account for the scheduling quantums from the target wakeup time and other work (like we were wrt the pipe write before)

src/vblankmanager.cpp

@@ -297,14 +297,14 @@ namespace gamescope
 			if ( !m_bArmed.exchange( false ) )
 				return;
 
-			uint64_t ulNow = get_time_in_nanos();
 
 			m_PendingVBlank = VBlankTime
 			{
 				.schedule = m_TimerFDSchedule,
-				.ulWakeupTime = ulNow,
+				.ulWakeupTime = m_TimerFDSchedule.ulScheduledWakeupPoint,
 			};
 #ifdef VBLANK_DEBUG
+			uint64_t ulNow = get_time_in_nanos();
 			fprintf( stderr, "wakeup: %lu\n", ulNow );
 #endif