Added simple polygon validity checks.

[julienjpk]

This commit fixes #31 and prevents the library from allocating invalid amounts of memory or
dereferencing invalid pointers. Obviously, we'd except users to not pass invalid polygons in the first place, but well... Better safe than sorry.

Please review the changes before merging if you plan to do so: I've made sure the code compiles but haven't tested it. There shouldn't be much room for error though.

[ivanfratric]

Thanks for submitting! A couple of comments:

• I am really not a fan of C++ exceptions. Instead of throwing an exception, I think the functions should return 0 instead (note that the return value of 0 indicates a failure for all triangulation and partition function).

• If the only check is the point count, then for performance reasons I think it's better to just do
  if(poly->numpoints < 3) return 0;
  instead a validate() call.

[julienjpk]

Done. I've kept a Valid method to avoid code duplication should the library ever include more tests (some maximum number of points, some checks on the coordinates, ...).

[ivanfratric]

In that case, what do you think about moving the Valid() method (for now) to polypartition.h and declaring it as inline? No performance loss in that case.

[julienjpk]

I've just made the change. Little note however: modern compilers have their own heuristics to determine whether or not a function should be inlined and are free to ignore an inline keyword whenever. In this case, it appears g++ does not decide to inline Valid, even at -O3 (which includes -finline-functions). The hint does make it happen though.

(Sorry for the useless commits)

[ivanfratric]

Thanks!