Rename glu libtess symbols. When enabled in WebKit, the internal_glu libtess

Rename glu libtess symbols.  When enabled in WebKit, the internal_glu libtess
symbols in skia will collide with the same internal_ implementation in WebKit.
Rename the glu entry points with an Sk_ prefix instead.  Also, unfork the
changes to the libtess source as much as possible (the same as was done in
http://trac.webkit.org/changeset/87178).

Review URL:  http://codereview.appspot.com/4536080/

git-svn-id: http://skia.googlecode.com/svn/trunk@1419 2bbb7eff-a529-9590-31e7-b0007b416f81