Bigger fringe bitmaps

[Yuri D'Elia <wavexx@thregr.org>]

I recently switched to a higher-resolution screen to basque in all the
hidpi glory.

And all is good. Pretty darn amazing for text rendering, in fact.

Except for the fringe.

Even at 16x16, these bitmaps are now indistinguishable from specks of
dust. At least on lucid the bitmaps do not get upscaled by setting a
larger fringe size.

I suspect upscaling with nearest-neighbor would be a possibility for the
moment, but it's the API surrounding the fixed bitmap sizes that doesn't
look future-proof to me.

I didn't think much of it at first, but it's while debugging with `gud'
that I'm really struggling and realizing how useful the fringe is.

Shouldn't we allow the fringe to be a single character instead of being
a bitmap? With it's own family on graphical displays?

Using a font for the fringe would allow character displays to
potentially show something useful by either using basic ascii symbols
like "-+" or using stuff from the drawing block.

On graphical displays we could use a separated dedicated family.
Font-awesome? **gasp**