multithreaded/SMP motion estimation

multithreaded/SMP motion estimation
multithreaded/SMP encoding for MPEG1/MPEG2/MPEG4/H263
all pthread specific code is in pthread.c
to try it, run configure --enable-pthreads and ffmpeg ... -threads <num>
the internal thread API is a simple AVCodecContext.execute() callback which executes a given function pointer with different arguments and returns after finishing all, that way no mutexes or other thread-mess is needed outside pthread.c

Originally committed as revision 2772 to svn://svn.ffmpeg.org/ffmpeg/trunk