calc content bounds for proper layer comp scaling

Fixes Issue #273 (Absolute Scale doesn't work with layer comps)

This is very similar to `Generator.prototype.getDeepBounds`. Did you take a look at that? Ideally this sort of functionality would be in core to go along with `getDocumentPixmap`. I'm not *dead set* on that, but it would be nice. Conceivably we could leverage `getDeepBounds` by just unioning the results of that call for each of the document's top-level layers?