Adds a job->frame_to_stop variable similar to the existing pts_to_stop parameter...

Adds a job->frame_to_stop variable similar to the existing pts_to_stop parameter, for ending an encode after a certain number of decoded frames.

CLI: Replaces --stop-at-pts and --stop-at-duration with a general --stop-at option. The argument is in the form unit:amount. So --stop-at duration:10 for a 10 second encode, --stop-at pts:300300 to end when that timestamp is reached, or --stop-at frame:1000 to stop after reading 1000 frames.

git-svn-id: svn://localhost/HandBrake/trunk@2286 b64f7644-9d1e-0410-96f1-a4d463321fa5