spans-compositor: After polygon intersection the fill rule is always non-zero

As it turns out due to the rules of polygon intersection, there is never
any overlapping spans so the choice is arbitrary. However, lets be
consistent with the rest of the code.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/cairo-spans-compositor.c

@@ -934,6 +934,8 @@ clip_and_composite_polygon (const cairo_spans_compositor_t	*compositor,
 		status = trim_extents_to_polygon (extents, polygon);
 		if (unlikely (status))
 		    return status;
+
+		fill_rule = CAIRO_FILL_RULE_WINDING;
 	    } else {
 		_cairo_polygon_fini (&clipper);
 	    }