Added overdraw visualization.

Ooooo, sounds good. Nice point about failing over and over.

So, with error tolerance in mind, for something like:
```
wtf.replay.graphics.OffscreenSurface.prototype.bindFramebuffer = function() {
  this.initialize();
  this.context.bindFramebuffer(goog.webgl.FRAMEBUFFER, this.framebuffer_);
};
```
Would I want to restructure that like this:
```
wtf.replay.graphics.OffscreenSurface.prototype.bindFramebuffer = function() {
  if (!this.ensureInitialized()) {
    return;
  }
  this.context.bindFramebuffer(goog.webgl.FRAMEBUFFER, this.framebuffer_);
};
```
? Could that be made shorter while still making the initialization cleaner and safer?