Add a thread pool for background job processing, and use it for OggDecoder.

The pool adjusts to the amount of required threads.
The implementation is as simple as possible, but should be enough
for future extensions.

This brings down decoder thread creation/destruction even more.

Also fix OpenALSystem::release() to close channels properly on shutdown,
otherwise it would deadlock with the pool, because it waits until all
threads have died off, which did not necessarily happen.

BBGE/FmodOpenALBridge.cpp

@@ -33,6 +33,7 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 
 #include "Base.h"
 #include "Core.h"
+#include "MT.h"
 
 #include "FmodOpenALBridge.h"
 
@@ -108,7 +109,7 @@ private:
     int freq;
 
 #ifdef BBGE_BUILD_SDL
-    SDL_Thread *thread;
+    Runnable *thread;
 #else
     #warning Threads not supported, music may cut out on area changes!
     // ... because the stream runs out of decoded data while the area is
@@ -282,11 +283,12 @@ bool OggDecoder::start(ALuint source, bool loop)
 
 #ifdef BBGE_BUILD_SDL
     stop_thread = false;
-    thread = SDL_CreateThread((int (*)(void *))decode_loop, this);
-    if (!thread)
+    thread = new FunctionRunnable(false, (void (*)(void *))decode_loop, this);
+    if (!core->threadpool.addJob(thread))
     {
-        debugLog("Failed to create Ogg Vorbis decode thread: "
-                 + std::string(SDL_GetError()));
+        delete thread;
+        thread = NULL;
+        debugLog("Failed to add Ogg Vorbis decoder to thread pool");
     }
 #endif
 
@@ -322,7 +324,8 @@ void OggDecoder::stop()
     if (thread)
     {
         stop_thread = true;
-        SDL_WaitThread(thread, NULL);
+        thread->wait();
+        delete thread;
         thread = NULL;
     }
 #endif
@@ -1313,10 +1316,9 @@ FMOD_RESULT OpenALSystem::release()
         for (int i = 0; i < num_channels; i++)
         {
             const ALuint sid = channels[i].getSourceName();
+            channels[i].stop();
             channels[i].setSourceName(0);
             channels[i].setSound(NULL);
-            alSourceStop(sid);
-            alSourcei(sid, AL_BUFFER, 0);
             alDeleteSources(1, &sid);
         }
         ALCdevice *dev = alcGetContextsDevice(ctx);