Possible calculation mistake in "instancing" example offset values

[sohamxeh]

In the "instancing" example, half_cell_w and half_cell_h appear to use 0.5 as numerator instead of 1.0. Normalized coordinates range is [-1.0, 1.0] (total width 2.0). The starting offset should be half of (2.0 / cols) = 1.0 / cols. Same for rows.