CPU usage too high, especially on old hardware

[bradfitz]

Now that #6 is fixed, I reached into the depths of my bins of Raspberry Pis and found the oldest one I had, a Model B Plus Rev 1.2.

Good news: it works!

Bad news: it uses 100% CPU in fbstatus... (draw: 1.491s, cp 111ms)

But looks like some of the text drawing can be optimized, keeping around the previous frame and shifting some lines up, and caching some pre-rendered glyphs (they're fixed-width w/o ligatures, constant background, etc).

[stapelberg]

Oops! Yeah, reducing CPU usage would be great. Improvements welcome :)