Pass the correct object to image load callbacks.

[Ventero]

Previously, the image object was passed on to onTileLoad regardless if it loaded successfully or not. In case an image failed to load, this would result in trying to draw a non-existing image onto the canvas, which (at least in Firefox) throws a DOM exception and results in the drawer being stuck mid-update.

[iangilman]

Great fix! Just a couple of comments:

• For clarity, could you use something like resultingImage instead of img inside complete()?
• Please update the documentation for loadImage's callback argument, to specify that the image will either be valid or null.

[Ventero]

Thanks for reviewing the pull request. I've added a commit which updates the documentation and fixes the variable name. I hope the updated documentation makes sense - since English isn't my first language, feel free to correct me if something's wrong. ;)

[iangilman]

Looks great!