fix "traceGpuCompletion" debug feature

eglCreateSyncKHR requires a call to glFlush which we were not doing.
fixed by moving the code above eglSwapBuffers(), which both
fixes the problem and gives us a slightly better idea of when
the GPU is done drawing.

Change-Id: Ic826ef1fe25a6247742c3d49d0cb69f4031e3593