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/Patch.h b/libs/hwui/Patch.h
index d5bbfa6..1af4bb7 100644
--- a/libs/hwui/Patch.h
+++ b/libs/hwui/Patch.h
@@ -31,7 +31,7 @@
 namespace android {
 namespace uirenderer {
 
-class TextureVertex;
+struct TextureVertex;
 
 ///////////////////////////////////////////////////////////////////////////////
 // 9-patch structures