revisit workaround for iOS allowTouchSnag problem

[pixelzoom]

Due to #29 (comment), scenery's "touch snag" feature was disabled (hopefully temporarily) for all instances of FunctionNode. When phetsims/scenery#547 is fixed, set allowTouchSnag:true.

[pixelzoom]

To enable, see FunctionNode line 34:

allowTouchSnag: false //TODO see https://github.com/phetsims/function-builder/issues/49

Either remove this line (preferred), or change the value to true.

[pixelzoom]

phetsims/scenery#547 is still unresolved, so can't revisit this yet.

[pixelzoom]

This is still blocked by phetsims/scenery#547.

[pixelzoom]

This issue is still blocked because phetsims/scenery#547 is still open (after 6+ years). So this will not be addressed for the 1.2 release #139.