ScaleManager updates and fixes

This fixes some regressions from the previous pull, and some issues not fixed.

As per @tjkopena 's notes, SHOW_ALL was quite broken in the last pull. It should now work again in full screen /AND/ work better when in another element, honoring it's current and max sizing (this might itself be a breaking change wrt. some expectations..) However, it still works in the window inner sizing when not in full screen so it will be affected by developer tools docked in the main view-port.

See - https://github.com/photonstorm/phaser/pull/1278#issuecomment-62234470

It also address full-screen centering in Firefox and IE, so it should fix - https://github.com/photonstorm/phaser/issues/1256

I've also been more diligent in outlining possibly unexpected changes.