Rewriting BaseWindingForPlane() in polylib.c from the ground up. The behavior

Rewriting BaseWindingForPlane() in polylib.c from the ground up.  The behavior
is totally unchanged, and I verified this by running extensive tests.  The
only difference is that the math precision is much much better now.
Performance should be better as well (but that is not tested).
This is a major milestone because it fixes two regression tests:
disappearing_sliver2 and sparkly_seam.
Improvements to math precision is ongoing and more improvements can probably
be made even after this patch.
I will update the README.txt files in the regression tests in a separate
commit.  This commit only includes the actual fixed code.

git-svn-id: svn://svn.icculus.org/gtkradiant/GtkRadiant/trunk@371 8a3a26a2-13c4-0310-b231-cf6edde360e5