May have fixed up barrycentric coloring for triangles.

- Main problem was probably just that I wasn't setting up the objects
  correctly in main.
- Also found a new equation for area of the triangle, which may be
  better, but I'm not sure. Either way, it seems to work now. I think.
Removed some unused code.

- Removed barrycentric transform matrix for triangles, I think the
  area calculation is easier.
- Removed the side lengths from the triangle object, not currently being
  used for anything.
Added gtk+ header files.
Removed gtk+ include files, use pkg-config instead.
Started building with GTK+

- Got SConstruct set up to use pkg-config to set options for building
  with installed GTK+.
- Launching a simply GTK window in main. It doens't do anything yet.