surfaceflinger: cache HWC client targets and buffers

Remember HWC client targets and buffers, and make sure we send each
unique slot/handle pair only once.  This allows the composer to
clone/register/retain each buffer only once.

Test: builds and boots
Change-Id: Ib485189043a9c132031e82d4d7380ace3bf9453d
diff --git a/services/surfaceflinger/DisplayHardware/DisplaySurface.h b/services/surfaceflinger/DisplayHardware/DisplaySurface.h
index d801bb3..cb08f08 100644
--- a/services/surfaceflinger/DisplayHardware/DisplaySurface.h
+++ b/services/surfaceflinger/DisplayHardware/DisplaySurface.h
@@ -25,6 +25,7 @@
 namespace android {
 // ---------------------------------------------------------------------------
 
+class Fence;
 class IGraphicBufferProducer;
 class String8;