Sizing issues on MacOS with dvisvgm option --bbox=preview

[karthink]

There are two options for dvisvgm to determine the bounding box of the preview image in the DVI.

• --bbox=preview, reading info provided by preview.sty: Works fine everywhere except for MacOS. On MacOS there is some slight clipping, < 10 px.

-- --exact-bbox, not sure how this works: This used to be slightly wrong everywhere but broke significantly in dvisvgm 3.1+.

I think it's clear we want to stick with --bbox=preview, but it would be nice if we could tweak increase the size of the bounding box a tiny bit on MacOS systems. I'm not sure how to do this yet.