-- Move vertically, but box the character in at the window boundaries.
moveY (x, y) offset = if y + offset < (-height / 2) + slimeHeight / 2
                      then (x, (-height / 2) + slimeHeight /  2)
                      else if y + offset > height / 2 - slimeHeight /  2
                      then (x, height / 2 - slimeHeight /  2)
                      else (x, y + offset)
moveY (x, y) offset = if y + offset < (-height / 2) + slimeHeight / 2
                      then (x, (-height / 2) + slimeHeight /  2)
                      else if y + offset > height / 2 - slimeHeight /  2
                      then (x, height / 2 - slimeHeight /  2)
                      else (x, y + offset)
moveY (x, y) offset = if y + offset < (-height / 2) + slimeHeight / 2
                      then (x, (-height / 2) + slimeHeight /  2)
                      else if y + offset > height / 2 - slimeHeight /  2
                      then (x, height / 2 - slimeHeight /  2)
                      else (x, y + offset)