(svn r17776) -Codechange: [SDL] make "update the video card"-process asynchronious. Profiling with gprof etc. hasn't shown us that DrawSurfaceToScreen takes a significant amount of CPU; only using TIC/TOC it became apparant that it was a heavy CPU-cycle user or that it was waiting for something.

The benefit of making this function asynchronious ranges from 2%-25% (real time) during fast forward on dual core/hyperthreading-enabled CPUs; 8bpp improvements are, in my test cases, significantly smaller than 32bpp improvements.
On single core non-hyperthreading-enabled CPUs the extra locking/scheduling costs up to 1% extra realtime in fast forward. You can use -v sdl:no_threads to disable threading and undo this loss.
During normal non-fast-forwarded games the benefit/costs are negligable except when the gameloop takes more than about 90% of the time of a tick.
Note that allegro's performance does not improve with this system, likely due to their way of getting data to the video card. It is not implemented for the OS X/Windows video backends, unless (ofcourse) SDL is used there.
Funny is that the performance of the 32bpp(-anim) blitter is, at least in some test cases, significantly faster (more than 10%) than the 8bpp(-optimized) blitter when looking at real time in fast forward on a dual core CPU; it was slower.
The idea comes from a paper/report by Idar Borlaug and Knut Imar Hagen.

src/thread/thread.h

@@ -68,6 +68,19 @@ public:
 	 * End of the critical section
 	 */
 	virtual void EndCritical() = 0;
+
+	/**
+	 * Wait for a signal to be send.
+	 * @pre You must be in the critical section.
+	 * @note While waiting the critical section is left.
+	 * @post You will be in the critical section.
+	 */
+	virtual void WaitForSignal() = 0;
+
+	/**
+	 * Send a signal and wake the 'thread' that was waiting for it.
+	 */
+	virtual void SendSignal() = 0;
 };
 
 #endif /* THREAD_H */