Add glFlush in surfaceflinger screenshot code.

After creating a syncKHR object with type EGL_SYNC_NATIVE_FENCE_ANDROID,
glFlush must be called before the EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute
is populated with a sync fd. We currently call eglDupNativeFenceFDANDROID
before issuing the flush.

Bug 18052459
Taken verbatim from matthew.k.gumbel@intel.com.

Change-Id: I3781d14f92862076e2bca7d27341a6dc6e7e3775
diff --git a/services/surfaceflinger/RenderEngine/RenderEngine.h b/services/surfaceflinger/RenderEngine/RenderEngine.h
index acbff9b..8d7529c 100644
--- a/services/surfaceflinger/RenderEngine/RenderEngine.h
+++ b/services/surfaceflinger/RenderEngine/RenderEngine.h
@@ -67,6 +67,7 @@
     virtual void dump(String8& result);
 
     // helpers
+    void flush();
     void clearWithColor(float red, float green, float blue, float alpha);
     void fillRegionWithColor(const Region& region, uint32_t height,
             float red, float green, float blue, float alpha);