public void set_fixed_height_from_font (int number_of_rows)
Sets the height of a renderer to explicitly be determined by the "font" and "y_pad" property set on
it. Further changes in these properties do not affect the height, so they must be accompanied by a subsequent call to this function.
Using this function is unflexible, and should really only be used if calculating the size of a cell is too slow (ie, a massive
number of cells displayed). If number_of_rows is -1, then the fixed height is unset, and the height is determined by
the properties again.