vt: terminal size can grow too big with small font

vt is using static buffers for on screen data, the buffer size is
calculated based on maximum supported screen size and 8x16 font.

When using hi-res graphics and very smaller than 8x16 font, we
need to be careful not to overflow static buffers in vt.

Testing: I did test by building smaller buffers than vt currently is using,
royger was testing on actual 4k capable hardware.

MFC after: 1 week
Tested by: royger