added glFinish after eglSwapBuffers
Added a glFinish() after eglSwapBuffers() calls in Rendering/OpenGL2/vtkEGLRenderWindow.cxx.
Currently, these frames use PBuffers, which are always single-buffered. See, e.g.,: https://forums.khronos.org/showthread.php/5242-EGL-double-buffering
Thus eglSwapBuffers() currently has no effect. I left it in place, in case Kitware wishes to change the implementation to no longer use PBuffers.
glFinish() ensures EGL render windows are synchronized properly, which was the intent of eglSwapBuffers(). This enables correct timing and benchmarking of EGL-rendered frames.