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.cpp b/services/surfaceflinger/RenderEngine/RenderEngine.cpp
index d5d5da8..a77e0e9 100644
--- a/services/surfaceflinger/RenderEngine/RenderEngine.cpp
+++ b/services/surfaceflinger/RenderEngine/RenderEngine.cpp
@@ -219,6 +219,10 @@
     drawMesh(mesh);
 }
 
+void RenderEngine::flush() {
+    glFlush();
+}
+
 void RenderEngine::clearWithColor(float red, float green, float blue, float alpha) {
     glClearColor(red, green, blue, alpha);
     glClear(GL_COLOR_BUFFER_BIT);
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);
diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 96a8adb..50968e7 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -3201,6 +3201,8 @@
                         EGLSyncKHR sync;
                         if (!DEBUG_SCREENSHOTS) {
                            sync = eglCreateSyncKHR(mEGLDisplay, EGL_SYNC_NATIVE_FENCE_ANDROID, NULL);
+                           // native fence fd will not be populated until flush() is done.
+                           getRenderEngine().flush();
                         } else {
                             sync = EGL_NO_SYNC_KHR;
                         }