... off-by one size in special cases

SIGNED_ROUND(), which is used by GimpToolRectangle, among other
things, used to round negative values which lie exactly between
two integers, i.e., -foo.5, down.  This could lead to the rectangle
being one pixel bigger than expected, in either dimension, when one
of its edges had a negative coordinate, and the opposite edge had a
positive coordinate.

Fix SIGNED_ROUND() to always round such values up, regardless of
sign.