Pango considers the font size in a PangoFontDescription to be in
points, and it converts to pixels by using the resolution set in
pango_cairo_context_set_resolution().

However, we were passing our plainly normalized Length values to
pango::FontDescription::set_size(), i.e. we were passing them in
pixels, not points.  Thus, Pango was applying extra scaling, and
messing up the final size.

This commit hardcodes a value of 72.0 for Pango's resolution.  This is
so that it will not apply any extra scaling on top of our
computations.  We are doing the inches-to-pixels conversion, instead
of letting Pango do it.

This commit adds 348-font-size-48dpi.svg which two pairs of squares
and text elements, which should both render at the same size:  one
pair is done in userspace pixels, and the other pair is done in
inches.

This commit also regenerates 310-font-size-at-48dpi-ref.png - it is
the only other file that uses absolute font sizes.

#348