void set_fixed_height_from_font(int number_of_rows
Sets the height of the renderer to a value determined by its current
number_of_rows specifies the number of rows of text
each cell renderer should be allocated. If
number_of_rows is -1, the fixed
height will be unset and the height will be determined by the properties
This method 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).