Added SmoothLinesLayer.

Uses opengl to draw the lines directly.  It looks "ok", but needs some
work.
Baseline Solution

There are still some small nuances to work out, like how to make the
coloring work properly when two colors overlap (this is a function of
the blending mode).  There is also a section in the book by Philip
Rideout where he uses an image for the line segments instead of
triangles, which might work out better (though handling multiple widths
will be a challenge).  I think its in good enough shape for the
tutorial pages to be put together, regardless, so this will be the
baseline.