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.