Add width="" attribute to <img> tags when using SVG image type. Allows
us to generate SVG files at larger size, then scale them down for
display.  Current factor that seems to work well is 1.15x.  We also need
to add $svg_upscale_magic_number because setting width/detail-width in
the configuration file results in PNG vs SVG images being displayed at
different sizes (size difference is 111.4%).