Branch/graph updates

@kadst43 @scottnc27603 @eric79

- Added variable scale so that the graph units are not always world units
- Added a function to set graph visibility without affecting the lines drawn on it (do not use the `visible` property for the graph - `visible` still works for the function lines themselves)
- Converted grid lines and axis lines to meshes
- Added internal (-ish, just using $) visibility property for use with the visibility toggle method for the graph
- Added depth order constants to use instead of `Infinity` to make the lines show over the axes and the axes over the grid plane.

Future TODO (not for this PR):
- Allow the developer to specify opacity / transparency levels for grid lines and axes (and make the transparency setting work with `renderTop`)

Used by: https://github.com/virtual-world-framework/mars-game/pull/135
Right.  Ignore me then. :)