gimp_pickable_contiguous_region_by_seed(): don't call
find_contiguous_region() with start coords outside the buffer, or it
will run into an infinite loop.
This maps threshold changes by moving the mouse more uniformly to the
growing/shrinking of the selected area, and the tools should work
perceptually anyway. Also, it makes things another 10 times as fast on
gamma-corrected drawables.
The functions don't need to know about sample_merged, in fact they
used the boolean only to either use the passed image or drawable and
otherwise ran the same code. The sample_merged logic belongs into
their callers (and sometimes not even there).
