Optimization of Vector2 instance allocation in ComputeCuspInfo

[AshrafSharf]

@jonathanolson - The issue #53 made significant difference in performance. I found one more method "computeCuspInfo" in Cubic.js which is creating quite a few vector2 instances.

  // t value for the cusp, and the related determinant and inflection points
    computeCuspInfo: function() {
      // from http://www.cis.usouthal.edu/~hain/general/Publications/Bezier/BezierFlattening.pdf
      // TODO: allocation reduction
      var a = this._start.times( -1 ).plus( this._control1.times( 3 ) ).plus( this._control2.times( -3 ) ).plus( this._end );
      var b = this._start.times( 3 ).plus( this._control1.times( -6 ) ).plus( this._control2.times( 3 ) );
      var c = this._start.times( -3 ).plus( this._control1.times( 3 ) );

      var aPerp = a.perpendicular();
      var bPerp = b.perpendicular();
      var aPerpDotB = aPerp.dot( b );

      this._tCusp = -0.5 * ( aPerp.dot( c ) / aPerpDotB );
      this._tDeterminant = this._tCusp * this._tCusp - ( 1 / 3 ) * ( bPerp.dot( c ) / aPerpDotB );
      if ( this._tDeterminant >= 0 ) {
        var sqrtDet = Math.sqrt( this._tDeterminant );
        this._tInflection1 = this._tCusp - sqrtDet;
        this._tInflection2 = this._tCusp + sqrtDet;
      }
      else {
        this._tInflection1 = NaN;
        this._tInflection2 = NaN;
      }
    },