Input is flagged as changing during prepImage even if HTML did not change

[stevengharris]

The prepImage function in markup.js ensures that images are resizable, selectable, and specify their width and height. In prepImage, we _callback('input') when done. Ultimately this causes the HTML to be fetched on the Swift side even in the case when prepImage made no changes to the HTML. This seems like a minor change, but if you are relying on callbacks to 'input' to indicate changes in the document, these are both unnecessary and may have side-effects for you.