The use of a per-buffer cached sampler is problematic, especially
since the cached sampler may cache stale data after the buffer is
modified.  Avoiding this requires careful use of
gegl_buffer_sample_cleanup().  On the one hand, when using
gegl_buffer_sample[_at_level]() for one-off samples in loosely-
coupled code, this effectively means calling
gegl_buffer_sample_cleanup() before/after each sample, defeating
the purpose of caching the sampler in the first place.  On the
other hand, when performing multiple samples in performance-
critical code, using a sampler object is much more efficient
anyway.

Remove the per-buffer cached sampler, and simply use a per-call
temporary sampler in gegl_buffer_sample[_at_level]().  Advise users
to use gegl_buffer_sampler_new[_at_level]() to create a sampler
object instead when performance is a concern.

Deprecate gegl_buffer_sample_cleanup(), which is not necessary
anymore, and keep it as a NOP for now.