If $font is not set, the default font is constructed from
font data linked into every libdraw binary. That process
was different from the usual openfont code, and so it was
not hidpi-aware, resulting in very tiny fonts out of the box
on hidpi systems, until users set $font.
Fix this by using openfont to construct the default font,
by recognizing the name *default* when looking for
font and subfont file contents. Then all the hidpi scaling
applies automatically.
As a side effect, the concept of a 'default subfont' is gone,
as are display->defaultsubfont, getdefont, and memgetdefont.