Previously, the number of threads used by
gegl_parallel_distribute_{range,area}() was proportional to the
number of elements to be processed, such that each thread processed
at least a user-provided minimal number of elements.  This,
however, fails to take into account the fact that each additional
thread lowers the effective cost of processing each additional
element, since processing is spread over more threads, and
therefore more elements are necessary to justify each additional
thread.

To find the optimal number of threads to use, we assume that the
cost of processing the elements is proportional to the number of
elements to be processed by each thread, and that each thread
additional incurs a fixed cost.  This cost is specified by a user-
provided parameter, relative to the cost of processing a single
element, replacing the old minimal per-thread element-count
paramter (it is expected, however, that this parameter will take
the same value as the old parameter; in particular, the minimal
number of per-thread elements for using two threads equals the the
cost.)

In other words, the cost of processing n elements, using t threads,
with a fixed realtive per-thread cost c, is assumed to be
proportional to:

  n / t + c * t

The number of threads, t, that minimize this cost, for a given per-
thread cost, c, is roughly proportional to the square root of the
number of elements, n.