need to draw lines for the grid and cursor box
private constants are just another field
need to get coordinate for mouse cursor
draw the cursor box
yellow for grid lines too faint on the grass
draw the grid rows in the right place

Is an off by two error better than an off by one error?
draw the grid columns in the right place

They were actually already in the right place, but it was completely
accidental.
use the new primitive extension method