In gimp_pickable_contiguous_region_by_color(), add a small epsilon
to the threshold value, to allow for small errors due to the input
color and pickable pixel-colors being converted to the common
format through different paths.
While we *could* special-case threshold == 0 when the input color
comes from the same pickable, as is the case for the select-by-
color tool, and perform an exact comparison in the original format,
in the more general case the input color can come from an arbitrary
source, such as a plug-in.
(cherry picked from commit a6c79770c3)
