Add support for plane-alpha in HWC

Change-Id: I218e7dd5f23de535aabce61e993002ab6cb46cdd
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 1e7ec21..05f7784 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -152,6 +152,7 @@
         virtual void setVisibleRegionScreen(const Region& reg) = 0;
         virtual void setBuffer(const sp<GraphicBuffer>& buffer) = 0;
         virtual void setAcquireFenceFd(int fenceFd) = 0;
+        virtual void setPlaneAlpha(uint8_t alpha) = 0;
         virtual void onDisplayed() = 0;
     };