[cairo] stroking tests and fixes

[Jeff Muizelaar]

Over at git://git.cairographics.org/~jrmuizel/cairo stroking-fixes I have the following
three changes:

  - Test closed dashed paths where the first and last sub-path do not
    join.
  - Modify degenerate-path test case to test dashed degenerate
    paths.
  - Fix degenerate-path stroking with dashed lines.

The first two just add test cases. The last one actually causes a
behaviour change. It makes stroking of degenerate paths at a dash offset
with dash-on draw the same way as normal degenerate paths. This also
makes cairo's behaviour consistent with ghostscript. 

I would have prefered to have gotten this in before the release so that
there wouldn't be any behaviour change post-release. Unfortunately, I
have been very busy lately and haven't gotten around to making it ready
until now.

Anyways, If the changes are ok, please pull.

-Jeff