Make it possible to track specified clip

Added a property to track_marker operator to be
able to render clip which name was passed to the
operator instead of clip from current clip editor
context.

Very much useful for automatic tests.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60974 954f8c5b-7b00-dc11-b283-0030488c597c