Skip reading existing images when running a full render

[jpenilla]

For full renders we can probably skip reading existing images and just overwrite. This would change behavior when using tools like Chunky's trim feature, but this is likely acceptable.

edit: Thinking about this more, it would only work for the most zoomed in image, which reduces the usefulness but would also allow applying it to radius renders that render an entire region. For zoomed out images only the initial read for any given full render would be skippable.