Fullscreen element sizes

Add new fields to EmscriptenFullscreenChangeEvent structure that report the new pixel sizes of the fullscreen element, as well as the whole screen size. These can be used to read and adjust the rendering canvas size appropriately when transitioning between fullscreen modes. Thanks to Joel Croteau for the suggestion at https://groups.google.com/forum/#!topic/emscripten-discuss/qaTrOXWv1Oc .

Bump version to 1.12.3 to update the cache after change to struct_info.json.