gaussian-blur: Make OpenCL filter choice match CPU

Don't activate OpenCL FIR version when CPU would use IIR.
gaussian-blur: Use CL_MEM_COPY_HOST_PTR

Using CL_MEM_USE_HOST_PTR seems to make the GPU angry.
operations: rewrite noise-spread.c with iterators/samplers

... instead of buffer_get/set.

Also fix a bad use of gegl_random_int_range