Fix virtual display nesting

This fixes the cycling rendering loop caused by nesting virtual
displays by preventing them from recomposing if their contents
haven't changed.

Bug: 12101046
Change-Id: I600365c0fd5d3ad93e04295d26cf9de177ffc79b
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index 01a9d2e..f750c6c 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -124,7 +124,9 @@
     int32_t                 getHwcDisplayId() const { return mHwcDisplayId; }
     const wp<IBinder>&      getDisplayToken() const { return mDisplayToken; }
 
-    status_t beginFrame() const;
+    // We pass in mustRecompose so we can keep VirtualDisplaySurface's state
+    // machine happy without actually queueing a buffer if nothing has changed
+    status_t beginFrame(bool mustRecompose) const;
     status_t prepareFrame(const HWComposer& hwc) const;
 
     void swapBuffers(HWComposer& hwc) const;