GstEnginePipeline: Fix file descriptor exhaustion by using shared thread pool

Replace per-pipeline QThreadPool with a shared static pool to prevent
file descriptor and thread exhaustion. Each GstEnginePipeline was creating
its own thread pool, leading to resource accumulation during frequent
pipeline creation/destruction (track changes, seeking, crossfade).

The shared pool is limited to 2 threads max since state changes are
typically sequential per pipeline. This prevents the crash in g_wakeup_new()
when creating eventfd for new thread event dispatchers.

Fixes #1687

src/engine/gstenginepipeline.cpp

@@ -42,6 +42,7 @@
 #include <QObject>
 #include <QCoreApplication>
 #include <QtConcurrentRun>
+#include <QThreadPool>
 #include <QFuture>
 #include <QFutureWatcher>
 #include <QMutex>
@@ -98,6 +99,23 @@ constexpr int kEqBandFrequencies[] = { 60, 170, 310, 600, 1000, 3000, 6000, 1200
 
 int GstEnginePipeline::sId = 1;
 
+QThreadPool *GstEnginePipeline::shared_state_threadpool() {
+
+  // C++11 guarantees thread-safe initialization of static local variables
+  static QThreadPool pool;
+  static const auto init = []() {
+    // Limit the number of threads to prevent resource exhaustion
+    // Use 2 threads max since state changes are typically sequential per pipeline
+    pool.setMaxThreadCount(2);
+    return true;
+  }();
+
+  Q_UNUSED(init);
+
+  return &pool;
+
+}
+
 GstEnginePipeline::GstEnginePipeline(QObject *parent)
     : QObject(parent),
       id_(sId++),
@@ -1848,7 +1866,7 @@ QFuture<GstStateChangeReturn> GstEnginePipeline::SetState(const GstState state)
     watcher->deleteLater();
     SetStateFinishedSlot(state, state_change_return);
   });
-  QFuture<GstStateChangeReturn> future = QtConcurrent::run(&set_state_threadpool_, &gst_element_set_state, pipeline_, state);
+  QFuture<GstStateChangeReturn> future = QtConcurrent::run(shared_state_threadpool(), &gst_element_set_state, pipeline_, state);
   watcher->setFuture(future);
 
   return future;