libsgui: Change Rect default constructor

The current Rect default constructor initializes to all zeroes. Change
it to initialize to an invalid Rect.

Modify all of the existing invocations of the default constructor to
maintain the existing behavior.

Bug 18173359

Change-Id: Ibbad076f1550a94f56d7b1cf5350071f2022e09a
diff --git a/include/private/ui/RegionHelper.h b/include/private/ui/RegionHelper.h
index 8c190dd..84eb100 100644
--- a/include/private/ui/RegionHelper.h
+++ b/include/private/ui/RegionHelper.h
@@ -72,7 +72,7 @@
     }
 
     void operator()(region_rasterizer& rasterizer) {
-        RECT current;
+        RECT current(Rect::EMPTY_RECT);
         do {
             SpannerInner spannerInner(spanner.lhs, spanner.rhs);
             int inside = spanner.next(current.top, current.bottom);