cogl-path: Don't try to union sub paths

cogl-path: Don't try to union sub paths

When drawing a path with only a single sub path, Cogl uses the
'even-odd' fill rule which means that if a part of the path intersects
with another part then the intersection would be inverted. However
when combining sub paths it treats them as separate paths and then
unions them together. This doesn't match the semantics of the even-odd
rule in SVG and Cairo. This patch makes it so that a new sub path is
just drawn as another triangle fan so that it will continue to invert
the stencil buffer. This is also much simpler and more efficient as
well as being more correct.

http://bugzilla.openedhand.com/show_bug.cgi?id=2088