Almost there ...

Spent some more time with my renderer yesterday evening, and there are now no more cracks. A case of using floor where I should have been using ceil [oh].

However, all is still not well. For some reason I have a 1 or 2 pixel offset when comparing my output to OpenGL's output. I don't think this is down to the accuracy loss when going to fixed point math, so I'll be taking a look at my matrix routines next.