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
vt_vga: Use Write Mode 0 to draw group of 8 pixels using 3 or more colors

This replaces the method based on Write Mode 3, which required reads
from the video memory to load the latches.

MFC after:	1 week