Track.insert_frame used to create markers with zero pattern size

In most cases it's harmles since this call was intended to be used
for importers only where pattern size was overriding after creation
anyway. But better don't allow things which will work unpredictable.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53626 954f8c5b-7b00-dc11-b283-0030488c597c
Motion tracking API change

Replace Tracks.add(count, frame) with Tracks.new(name, frame)
which will return newly created track. Before there was no
reliable way to get newly created tracks.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53627 954f8c5b-7b00-dc11-b283-0030488c597c