closed #3698: Wrong display when dragging window through retina and non-retina screen.

1) On retina screen, the `frameZoomFactor` should be multiplied by 2.
2) The event of dragging window though retina and non-retina screen should be triggered by `onGLFWframebuffersize` (framebuffersize changed).
3) Adding EGLView::isRetina() method for Mac.