Improvement: speed up draw_line() for a special case.

```
draw_rect and draw_circle both rely heavily on draw line now when filling.  
However these instances are only ever with horizonal lines; and so a special 
case will be added that will edit pixels by the byte vs one at a time.

```

Original issue reported on code.google.com by `mdmetzle@gmail.com` on 2 Nov 2010 at 2:39
