Sorted out the barriers/corners trouble left by r5844. No wonder

Sorted out the barriers/corners trouble left by r5844. No wonder
Jacob wasn't able to find a satisfactory fix: the whole area was a
horrid mess. Fortunately, the reason it was a horrid mess was
because the Net drawing routines predated the introduction of clip()
in the frontend interface, and it turns out that clip() makes it
possible to do all this more easily and better. So, a complete
rearchitecting of barrier corners: the corner flags in the
`barriers' array are now gone (and good riddance), and corner
information is computed on the fly so as to take into account the
moving grid edges. In the process I've also updated the corner
mechanism so that a barrier `corner' (really endpoint) is drawn at
the end of _every_ barrier, not just where two meet. This has
changed the appearance of a single isolated barrier, to what I would
have wanted it to look like in the first place but achieving it
without clip() was just too fiddly.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5846 cda61777-01e9-0310-a592-d414129be87e