Add an execute2 function that is more flexible and allows to use parallel

Add an execute2 function that is more flexible and allows to use parallel
processing with jobs > threads without wasting too much memory.
It also avoids needing a separate int array when the only additional data
the jobs needs is a single int running from 0 to count-1.

Originally committed as revision 20210 to svn://svn.ffmpeg.org/ffmpeg/trunk