This changes them from a series of line segments to a path (continuous segments).
The primary motivation for this change was to fix a regression introduced by upgrading to wxWidgets 3.1.3 where the bottom line of the turtle was white on startup and in a few other places. I didn't track down what caused this, but instead noticed that using wxDC::DrawLines() didn't have this problem.
I was hoping this would also improve drawing time when the turtle is shown, as there are fewer points to calculate and draw, but BENCHMARK showed no improvement.
Note that, in PERSPECTIVE mode, the turtle can be partially on-screen, which means that the turtle's shape is not continuous, but may be broken line segments. Because of this, the turtle retains the old segment-based drawing behavior as a fall-back.
