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);