Add option to set image margin

[jdugge]

It can sometimes be helpful to extend the texture of a model beyond the island edges. That way, even if the cuts aren't completely precise, no white gaps become visible. This commit makes the rd.bake_margin property available in the UI.

[addam]

I have personally removed this option a few years ago because the margin-ed images somewhat confuse the shape of the net, and because I try to keep all the switches and buttons to a minimum. Also, paper is white on cross-cut, so I usually paint over the edges anyway. Have you tried making some models with this margin, are you sure it helps?

[jdugge]

In my experience, having the margins helps a lot. However, this is probably at least in part due to the fact that I use invisible edge lines because I cut the models using a digital cutting machine, and without the margin the pixelated edge of the texture always shows:

Maybe it would be possible to tuck away such specialised options in an "Advanced Settings" section that is collapsed by default?