Implement interp.redraw equivalent

[tmickel]

In Scratch 2.0, there's a global interpreter flag that tracks whether a block has triggered a sprite's drawn representation to become out-of-date. E.g, a "move 10 steps" block would trigger redraw, but a "set var to 20" wouldn't. The value of this flag determines whether or not a loop yields at the end of a frame (if the interpreter isn't in turbo mode). We need an equivalent flag in scratch-vm, and we can also use something similar to determine whether we should call the renderer's draw method.

https://github.com/LLK/scratch-flash/blob/master/src/interpreter/Interpreter.as#L84