vt(4): Store a rectangle for the drawable area, not just the top-left corner

This allows backends to verify they do not draw outside of this area.
This fixes a bug in vt_vga where the text was happily drawn over the
right and bottom margins, when using the Gallant font.

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270613 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f