grid.c: fix size miscalculation in Floret tiling.

A Floret grid of height h usually alternates columns of h hexagonal
florets with columns of h-1. An exception is when h=1, in which case
alternating columns of 1 and 0 florets would leave the grid
disconnected. So in that situation all columns have 1 floret in them,
and the starting y-coordinate oscillates to make the grid tile
sensibly.

However, that special case wasn't taken account of when calculating
the grid height. As a result the anomalous extra florets in the
height-1 tiling fell off the bottom of the puzzle window.