Don't grow the grid if not needed, (shift tiles instead)

Don't grow the grid if not needed, (shift tiles instead)

At least, shifting of the tiles is what the user experiences. Within
the code, the concept is simpler: We don't have a grid size at all
anymore, but simply compute the grid bounds that we want whenever
things change. The bounds are made to just fit the tiles (plus one
extra row/column on each side) and never smaller than the original
size chosen to fill the screen.