Work toward T49043, with a side of client vertex arrays.
Not a straightforward port from glVertex to immVertex since Gawain needs
to know how many vertices we'll be drawing *before* we start drawing.
Fixed these not-so-great aspects of grid drawing:
- coarse grids would draw atop some lines from the finer grids
- visible axes would draw atop lines from coarse grid
- axes were drawn even if they weren't in view
- terrible misuse of vertex arrays
- each line issued its own draw call
New code draws each line exactly once. The entire grid is one draw call.
Bonus: I had to / got to learn how the units system works!
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