Add a method to compute the font size needed to make text rendered
by FreeTypeTools or MathTextUtilities fill a specified rectangle.
The documentation of vtkTextMapper::GetConstrainedFontSize is
clarified, too.

Change-Id: I39de26b5173153eaec9e8f5f2857fe9964bad768

s constrained -- add note about updated docs for tmapper

Change-Id: Ib8c042285aabc719fe57825c05d85b6ab2b59c4b