In GimpSizeEntry, the value corresponding to 0%, as per
gimp_size_entry_set_size(), may be non-zero.  This works correctly
when using the size entry in percentage mode, but not when using
precentage as part of arithmetic.

Fix this by adding an 'offset' parameter to eevl's unit-resolution
callback, which can be specifies a constant value to add as part
of unit conversion, after scaling the converted value by the
conversion factor.  In GimpSizeEntry, use this parameter to offset
percentages by their lower bound.