Rename gegl_operation_get_min_threaded_sub_size() to
gegl_operation_get_pixels_per_thread().  This function returns the
(very hypothetical, ATM) cost of using each additional thread for
processing the operation, realtive to the cost of processing a
single pixel.  As before, it simply returns 64 * 64 for now.  This
function is meant to be used for the thread_cost parameter of
gegl_parallel_distribute_area().

Use the new function in the auto-threaded operation classes, and in
manually-threaded operations using gegl_parallel_distribute_area().
Additionally, use the function in manually-threaded operations
using gegl_parallel_distribute_range(), by dividing its result by
the relevant dimension of the processed area.