Fixed bug where Cone ScenePrimitive would be missing one or more faces.

Bug occurred when creating cone with radius 1 or greater eg:
new ScenePrimitive(ScenePrimitive::TYPE_CONE, 3.5,2.0, 12);

Original algorithm assumes that lastx > -1 for all iterations
except first pass of vertex calcuations.  In fact, lastx has a lower bound
of -1*radius, thus will be <= -1 often when radius is -1 or greater.

In some examples, I needed to adjust cone->setYaw(90) so that the missing
faces were presented to the camera.