Web: the fullscreen button placement is slightly off #1358

The button to toggle fullscreen in the web browser when running in windowed mode is slightly off. We could pass the rectangle of the desktop fullscreen button that we have in Godot for the Win, Mac, and Linux version to the browser to calculate the relative placement that this button should take. or we could directly calculate the relative placement of the desktop button in the app and just pass the calculated information to the browser as well, whichever works.