Implement clip to document as image post processing

This is ready to go, and is required by https://github.com/adobe-photoshop/generator-assets/pull/315

This adds the options to calculate the area to extract if you want to clip to the document bounds (or any clipTo bounds you want to use)
The method `getDeepBounds` is supposed to operate on the layer representation provided by getDocumentInfo / used by imageChanged events. This code implies you're augmenting the layer representation (I assume) in generator-assets.

In other words, these changes break the assumption that generator-core is not dependent on generator-assets. We cannot assume that generator-assets will always be running in generator-core.

I realize that getDeepBounds isn't documented. But, it's a public function, so we certainly can't add external dependencies to that function. Our options are:

1. Remove those dependencies
2. Remove the `getDeepBounds` method entirely (i.e. move it to generator-assets) and increment the "major" version number if generator-core. Document this breaking change.

I would prefer to do option 1