Provided .svg button fallback

Haven't been able to determine why the .svg files included in the .zip file for the project don't, you know, draw to Canvas when ordered to on seemingly everyone else's computer except mine, but I just updated the game to provide a quick-and-dirty fallback option of a blue square of identical size and location labeled with "Roll" or "+" so people have something they can see to click on. (The spaces of canvas were still clickable as if the buttons were present before, but the buttons were entirely invisible.)