event/DeferEvent: split the thread-safe version into new class InjectEvent

src/output/plugins/AlsaOutputPlugin.cxx

@@ -37,7 +37,7 @@
 #include "util/ConstBuffer.hxx"
 #include "util/StringView.hxx"
 #include "event/MultiSocketMonitor.hxx"
-#include "event/DeferEvent.hxx"
+#include "event/InjectEvent.hxx"
 #include "event/Call.hxx"
 #include "Log.hxx"
 
@@ -55,7 +55,7 @@ static constexpr unsigned MPD_ALSA_BUFFER_TIME_US = 500000;
 class AlsaOutput final
 	: AudioOutput, MultiSocketMonitor {
 
-	DeferEvent defer_invalidate_sockets;
+	InjectEvent defer_invalidate_sockets;
 
 	/**
 	 * This timer is used to re-schedule the #MultiSocketMonitor