add a loading progress bar

[getify]

want: animated svg (an svg with css animation embedded in it) for a progress bar... probably just a simple spinner of some sort. should be in place with an <img> tag by default, with the scene canvas hidden, until game has loaded and is ready to go.

[getify]

thinking the spinner should be like a plane propeller spinning (kinda slowly) and maybe it has that "wagon wheel" visual effect of showing the word "loading" in between the propellers.