I'm a bit on the fence here about whether we should create a whole new function (e.g. ```Generator.prototype.getBounds(documentId, layerId)```). If we did that, we could more easily swap out the implementation when it gets faster. And, we could use the fast approach (access doc info) for layers where we knew it would work.

On the other hand, the developer will think of getPixmap as being expensive, so they might think of the bounds only version being expensive as well.

Also, most of the time, the users will be getting bounds directly from the doc info anyway. And some day, hopefully that info will be correct.