Made dlib::thread_pool use std::thread and join on the threads in

thread_pool's destructor.  The previous implementation used dlib's global
thread pooling to allocate threads to dlib::thread_pool, however, this
sometimes caused annoying behavior when used as part of a MATLAB mex file.

dlib/threads/thread_pool_extension.cpp

@@ -19,12 +19,11 @@ namespace dlib
         we_are_destructing(false)
     {
         tasks.resize(num_threads);
+        threads.resize(num_threads);
         for (unsigned long i = 0; i < num_threads; ++i)
         {
-            register_thread(*this, &thread_pool_implementation::thread);
+            threads[i] = std::thread([&](){this->thread();});
         }
-
-        start();
     }
 
 // ----------------------------------------------------------------------------------------
@@ -60,7 +59,10 @@ namespace dlib
             task_ready_signaler.broadcast();
         }
 
-        wait();
+        // wait for all threads to terminate
+        for (auto& t : threads)
+            t.join();
+        threads.clear();
 
         // Throw any unhandled exceptions.  Since shutdown_pool() is only called in the
         // destructor this will kill the program.

dlib/threads/thread_pool_extension.h

@@ -14,6 +14,7 @@
 #include "../smart_pointers_thread_safe.h"
 #include "../smart_pointers.h"
 #include <exception>
+#include <thread>
 
 namespace dlib
 {
@@ -126,7 +127,7 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
-    class thread_pool_implementation : private multithreaded_object
+    class thread_pool_implementation 
     {
         /*!
             CONVENTION
@@ -474,6 +475,8 @@ namespace dlib
         signaler task_ready_signaler;
         bool we_are_destructing;
 
+        std::vector<std::thread> threads;
+
         // restricted functions
         thread_pool_implementation(thread_pool_implementation&);        // copy constructor
         thread_pool_implementation& operator=(thread_pool_implementation&);    // assignment operator