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/Core.h

@@ -42,6 +42,8 @@ BUILD_LINUX
 
 #include "DarkLayer.h"
 
+#include "MT.h"
+
 /*
 #ifdef BBGE_BUILD_WINDOWS
 	#include "Joystick.h"
@@ -1313,6 +1315,8 @@ public:
 
 	int tgaSave(const char *filename, short int width, short int height, unsigned char	pixelDepth, unsigned char	*imageData);
 
+	ThreadPool threadpool;
+
 protected:
 
 	std::string fpsDebugString;