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/RenderBufferCache.h b/libs/hwui/RenderBufferCache.h
index af8060f..6c668b0 100644
--- a/libs/hwui/RenderBufferCache.h
+++ b/libs/hwui/RenderBufferCache.h
@@ -78,11 +78,11 @@
 private:
     struct RenderBufferEntry {
         RenderBufferEntry():
-            mBuffer(NULL), mWidth(0), mHeight(0) {
+            mBuffer(nullptr), mWidth(0), mHeight(0) {
         }
 
         RenderBufferEntry(GLenum format, const uint32_t width, const uint32_t height):
-            mBuffer(NULL), mFormat(format), mWidth(width), mHeight(height) {
+            mBuffer(nullptr), mFormat(format), mWidth(width), mHeight(height) {
         }
 
         RenderBufferEntry(RenderBuffer* buffer):