Add SetCellStretchToFit() and ResetCellStretch functions

[nickjwhite]

The SetCellStretchToFit function can be used to set horizontal stretching
of text to fit the full width of a cell. This is particularly useful when
trying to precisely line up text in a cell with an OCR image below it, and
as a pleasant side-effect can also prevent PDF readers from assuming that
words in cells on the same line with some space between them are in fact on
different lines when copy-pasting the text.

The ResetCellStretch function resets the cell stretch value to the default,
which is no stretch (100%).

This uses the Tz text state operator described in section 9.3.1 of the PDF 3200-1:2008 specification.

An alternative (php) implementation http://www.fpdf.org/en/script/script62.php wraps the Cell() method instead, but I prefer the stateful approach here, as it's cleaner and simpler.

This is super useful for me in creating searchable PDFs from page images (using invisible text), as it means I can set the text in each Cell to take the whole space allocated, and as the OCR engine gives the exact size of each text box this makes the whole thing very accurate.