Adding diagrams for the blog post.
Improved efficiency in DebugLinesLayer.

The BIG problem with the way DebugLinesLayer worked is that it kept ALL
the lines it needed to draw internally (not a problem) and then drew
them every draw() call (big problem).  The answer is to create a
RenderTexture and draw to this instead.  Add it as a layer to the scene
and erase the lines by clearing the texture when needed.

TRADEOFF:  Costs memory to hold a render texture for the whole screen
vs. minimal hit to performance for redrawing the entire texture with
blending (negligble).
Added code for calculating velocities and widths.

Experimental code.  Not sure I like the results.  Looks "OK", but I
wonder if I could do better with the equivalent of a PID (or at least
P+I) loop.
Width generation response improved.

I added a feedback loop (proportional only) around the width and it has
something close to the desired response.  When the difference is small
between the estimated width and the current width, it changes very
little or not at all.  When the difference is great (i.e. fast stroke),
the width grows quickly.  With the current settings, a fast swipe has
an estimate for the width around 14.5.  I set the upper limit to 10 so
that it could "max out" when doing fast draws, but still handle slow
speeds well.  Some tweaking necessary, but path looks promising.