Vector Rendering Update Error

[Denz1994]

While working with @samreid in Masses and Springs we discovered that the startPoint and subpath.getFirstPoint() are not equal and thus firing an error.

Steps to reproduce:

1. Start Masses and Springs sim
2. Select Vector Screen
3. Select Velocity Vector in visibility panel
4. Drag mass and attach to spring so it oscillates
5. Stop spring using stop sign button next to the spring hanger.
6. Drag mass off of spring
7. Error should be fired

Checking the startPoint and subpath.getFirstpoint() shows that they aren't equal in this piece of code.

Shape.js

        if ( subpath.isDrawable() ) {
          // since the commands after this are relative to the previous 'point', we need to specify a move to the initial point
          var startPoint = subpath.segments[ 0 ].start;
          assert && assert( startPoint.equalsEpsilon( subpath.getFirstPoint(), 0.00001 ) ); // sanity check
          string += 'M ' + kite.svgNumber( startPoint.x ) + ' ' + kite.svgNumber( startPoint.y ) + ' ';

Any insight would help. Thanks @jonathanolson.

[samreid]

I committed a workaround for MassNode. You can uncomment any of the if ( Math.abs( springForce ) < 1E-6 ) { lines to get the bug back.

[jonathanolson]

The mutation of Shape points triggered the assertion (since the assertion was written before the mutability was supported). There's no good way for the assertion to help (or for Kite to internally track those point changes), so I've removed the assertion.