According to the C++ standard, "this" is never NULL, so checks
of the form "if(!this)" can be legally optimized out. This
breaks SolveSpace on GCC 6, and probably on other compilers and
configurations.
Instead of always using two points on every curve, with a hack for
some cubics edge case, use three points on the first iteration and
one point on every further iteration. This both faster and more
correct.
The SolveSpace top-level directory was getting a bit cluttered, so
following the example of numerous other free-software projects, we move the
main application source into a subdirectory and adjust the build systems
accordingly.
Also, got rid of the obj/ directory in favor of creating it on the fly in
Makefile.msvc.