Fix merged operation clipping

bug:10745870

Missing 'const' meant MergingDrawBatch would never clip anything.

Change-Id: Ia6367eff94cf5f437efafbc3ba7f0da102ffd956
diff --git a/libs/hwui/DeferredDisplayList.cpp b/libs/hwui/DeferredDisplayList.cpp
index fd9257a..c0b77c7 100644
--- a/libs/hwui/DeferredDisplayList.cpp
+++ b/libs/hwui/DeferredDisplayList.cpp
@@ -249,7 +249,7 @@
         return true;
     }
 
-    virtual void add(DrawOp* op, DeferredDisplayState* state, bool opaqueOverBounds) {
+    virtual void add(DrawOp* op, const DeferredDisplayState* state, bool opaqueOverBounds) {
         DrawBatch::add(op, state, opaqueOverBounds);
 
         const int newClipSideFlags = state->mClipSideFlags;