#74 Grid should scale with viewing depth

[Richard]

The new way of drawing the grid is nice and useful, but
it's very bad on the eyes because of the interfering
patterns.
(especially on a low resolution of 800x600 it's worse)

This can be solved by (visually) adding/removing units
when zooming in or out of the 3d views, like 3DS MAX does.

See the attached screenshot for more.

[Richard]

Viewport patterns