Translate origin to center before laying out text

Move the origin to the center of the image in preparation for allowing
the text to be rotated.
Allow watermark text to rotate

Add command-line argument (`-r') for specifying a rotation (in degrees)
to rotate the watermark text.
Add symbolic rotation specs for diagonals

Add support for "ldiag" and "rdiag" as rotation arguments.  These will
rotate the text to the left (counter-clockwise) diagonal and right
(clockwise) diagonal respectively.