Sync zoom point/rectangle/scale methods...

[sdlime]

At the moment MapScript supports the zooming convenience methods in a different
way than the CGI. Not a huge deal but it would be nice to get those out of
MapScript and into C and then the CGI. There looks to be a lot of repetition in
those zoom methods. For instance, each one does the max extent checking. One
function might be used instead.

Steve