Initially I wanted to make the font code separate from
the main wterm executable in the hope that it would be
reusable. However, we do need to be able to reach in
to both the font implementation and the SDL texture
implementation in order to capture the glyph data
into textures, so let's just embrace it all being
in the same crate.
Trigger x-scaling based on the x_advance rather than the bitmap width.
Also account for the unicode width of the fragment when scaling.
The use of bearing_x was incorrect; we were subtracting it rather
than adding it and making the glyphs look terrible at smaller sizes.
