Add overrides and switch to nullptr keyword

Changes generated with clang-modernize.

Additionally, fixed some struct-vs-class usage to make clang happy.

Change-Id: Ic6ef2427401ff1e794d26f21f7b44868fc75fb72
diff --git a/libs/hwui/RenderProperties.h b/libs/hwui/RenderProperties.h
index be43e60..f0e22d6 100644
--- a/libs/hwui/RenderProperties.h
+++ b/libs/hwui/RenderProperties.h
@@ -188,7 +188,7 @@
         if (matrix) {
             mStaticMatrix = new SkMatrix(*matrix);
         } else {
-            mStaticMatrix = NULL;
+            mStaticMatrix = nullptr;
         }
         return true;
     }
@@ -203,7 +203,7 @@
         if (matrix) {
             mAnimationMatrix = new SkMatrix(*matrix);
         } else {
-            mAnimationMatrix = NULL;
+            mAnimationMatrix = nullptr;
         }
         return true;
     }
@@ -571,7 +571,7 @@
 
     bool hasShadow() const {
         return getZ() > 0.0f
-                && getOutline().getPath() != NULL
+                && getOutline().getPath() != nullptr
                 && getOutline().getAlpha() != 0.0f;
     }