Mark non-inplace Region operations WARN_UNUSED

Since the Java side equivalents operate in-place it can
be easy to assume Region.translate(x,y) will have some
effect whereas it is in fact a no-op unless the return
value is stored. Use WARN_UNUSED to help developers with
this.

Test: None
Change-Id: Idfd5b8b68dc144acaae62bd732eecdac7a745197
diff --git a/libs/ui/include/ui/Region.h b/libs/ui/include/ui/Region.h
index 7788452..68b60fc 100644
--- a/libs/ui/include/ui/Region.h
+++ b/libs/ui/include/ui/Region.h
@@ -25,6 +25,8 @@
 #include <ui/Rect.h>
 #include <utils/Flattenable.h>
 
+#include <android-base/macros.h>
+
 namespace android {
 // ---------------------------------------------------------------------------
 
@@ -93,11 +95,11 @@
             Region&     subtractSelf(const Region& rhs, int dx, int dy);
 
             // these translate rhs first
-    const   Region      translate(int dx, int dy) const;
-    const   Region      merge(const Region& rhs, int dx, int dy) const;
-    const   Region      mergeExclusive(const Region& rhs, int dx, int dy) const;
-    const   Region      intersect(const Region& rhs, int dx, int dy) const;
-    const   Region      subtract(const Region& rhs, int dx, int dy) const;
+    const   Region      translate(int dx, int dy) const WARN_UNUSED;
+    const   Region      merge(const Region& rhs, int dx, int dy) const WARN_UNUSED;
+    const   Region      mergeExclusive(const Region& rhs, int dx, int dy) const WARN_UNUSED;
+    const   Region      intersect(const Region& rhs, int dx, int dy) const WARN_UNUSED;
+    const   Region      subtract(const Region& rhs, int dx, int dy) const WARN_UNUSED;
 
     // convenience operators overloads
     inline  const Region      operator | (const Region& rhs) const;