change the model of curves approximation to one that

change the model of curves approximation to one that
a) almost matches the previous one, and
b) makes more sense, mathematically
now the inexactness of a patch is defined as the maximum over the inexactness of its quadratic splines;
the inexactness of such a quadratic spline is defined as the area between the spline and the line between its end points;
as that area is hard to calculate, the triangle area of the three control points is taken instead (which obviously is larger than the area between spline and the line between the end points, so this actually is a pessimistic approach to get some nicely defined tesselation)

not much should change for the user, though, but flat patches are now properly detected and handled

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@8817 d7cf8633-e32d-0410-b094-e92efae38249