Fixup synthetic BoldItalic variants.

[flowchartsman]

Thanks to #5 Greybeard now has synthesized BoldItalic variants generated from the italic strikes using mkbold-mkitalic. Unfortunately, the algorithm does not take bounding boxes into account, so anywhere a thickened glyph would run over the edge (usually by a single pixel), it simply cuts off. This isn't too bad, but it's bothering me. I'm sure there's a way to expand the bounding boxes dynamically and to use the bdf analogue of kerning to account for it, but it requires some investigation and, likely, digging into the annals of old xfont utilities.

[flowchartsman]

Some of the glyphs also have some empty pixels as artifacts of the process. These stand out, and should be dealt with as well. I can't think of a good way to do this other than straight up manual patching, which would be simply awful to do for every glyph Greybeard supports, but might not be too bad for a more limited, more visible subset, such as ASCII.

If I was feeling particularly sassy, I might even backport the synthetic versions to the original .bdf templates, which would probably make dealing with similar characters easier, but let's not get ahead of ourselves here.