display a grid to show the ground level

[sbarthelemy]

displaying a grid would be useful to:

• have a perception of the ground level and the vertical direction
• have perception of the distance

However this would add more parameters to tune:

• the size of the grid
• the spacing of the grid

and in many cases a shapes.Box would still be needed for collisions with the ground, and would hide the grid.

[sbarthelemy]

Practically the grid would be drawn with lines, which do not work with current osg-swig version (2.6). There is a patch in Seb and Joe mailboxes that implements it with a semi-transparent plane, but which was not merged.