Add sleep to avoid 100% busy CPU loop in PhysicsServerExample

Added btClock::usleep
Fix broken TinyRenderer example code.

examples/Utils/b3Clock.h

@@ -28,6 +28,10 @@ public:
 	/// the Clock was created.
 	double getTimeInSeconds();
 
+	///Sleep for 'microSeconds', to yield to other threads and not waste 100% CPU cycles.
+	///Note that some operating systems may sleep a longer time.
+	static void usleep(int microSeconds);
+
 private:
 	struct b3ClockData* m_data;
 };