Fix tiling on QCOM GPU

Rename Caches::startTiling parameter from opaque to discard to
make it clearer what its role is. Tweak calls to startTiling
to preserve the buffer when needed and discard it when possible.

Change-Id: If7f8ff19003f79f36885a0a7207cc61901f637d2
diff --git a/libs/hwui/Caches.h b/libs/hwui/Caches.h
index ae188be..69c81c8 100644
--- a/libs/hwui/Caches.h
+++ b/libs/hwui/Caches.h
@@ -215,7 +215,7 @@
     bool disableScissor();
     void setScissorEnabled(bool enabled);
 
-    void startTiling(GLuint x, GLuint y, GLuint width, GLuint height, bool opaque);
+    void startTiling(GLuint x, GLuint y, GLuint width, GLuint height, bool preserve);
     void endTiling();
 
     /**