vt(4): Intialize drawable area rectangle each time a font is loaded

This also fixes a problem where early in boot, the area was zero,
leading to nothing displayed for a few seconds.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270618 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f