Tank-throbs

I added a "throbber" cursor on top of the tank so you can visually confirm which tank is selected. Right now there's only one friendly tank, but soon there will be more. To animate the cursor I place a transparent box over the tank and apply numeric interpolation to the alpha channel.

Here is a screen shot with cursor off:

Here it is again with cursor on, e.g. the tank is selected:

There was a small issue with this: moving the tank while the cursor throbs looks weird.

So I turned that off while the tank is moving. That in turn makes it look weird as the cursor "stutters" between rapid tank moves. 

Luckily for me, it's only noticeable when I am coding the tank scene on its own in Godot. When the tank is superimposed on the world/background, the oddness isn't visible. I'll call that a win, and move on.