Blender Git Loki

draw ortho grid with new immediate mode

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!