Document PillarboxPlayer APIs as supported on the main thread only

Sources/Player/Player.docc/PillarboxPlayer.md

@@ -8,6 +8,8 @@ Create engaging audio and video playback experiences.
 
 ## Overview
 
+> Warning: PillarboxPlayer APIs are currently meant to be used from the main thread only. Calling APIs from background threads is not supported and leads to undefined behavior.
+
 The PillarboxPlayer framework provides a complete toolbox to add advanced audiovisual media capabilities to your app.
 
 Play content easily with a ``Player`` and display its content in the standard AVKit user interface with ``SystemVideoView``, or build an entirely custom user interface starting from a simple ``VideoView``. Play any kind of content from any source by implementing your own ``PlayerItem``s and ``Asset``s, no matter where your content comes from or how it must be played. Track playback with your own ``PlayerItemTracker``, whether for analytics purposes or Quality of Service (QoS) data collection.

Sources/Player/Tracking/CurrentTracker.swift

@@ -19,12 +19,14 @@ final class CurrentTracker {
         item.enableTrackers(for: player)
 
         player.propertiesPublisher
+            .receiveOnMainThread()
             .sink { properties in
                 item.updateTrackerProperties(properties)
             }
             .store(in: &cancellables)
 
         item.metricLog.eventPublisher()
+            .receiveOnMainThread()
             .sink { event in
                 item.receiveMetricEvent(event)
             }