Add support for margins on generated images

[jackwilsdon]

It'd be nice to have this as the default - maybe generating images with a "1 block" margin like so (1px border added to clarify margin);

This does raise the issue of "how do we scale images" now, as currently if you ask for a 512x512 image then we draw blocks of (512/8)x(512/8), which wouldn't be possible if a margin is added.

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Alternatively we could discard the outer 1 block border like so, which does avoid the issue of scaling;

The issue now is that we've discarded over 40% of the source image by removing the surrounding 28 blocks! 😱