buffer-cl: remove one stack frame from gegl_buffer_cl_cache_flush
perf: add a test measuring sampler performance
samplers: set OFFSET_0 of lohalo/nohalo to 7

For affine like buffer sampling a 5.23x speed improvement for nohalo and 2.45x
speed improvement for lohalo. This tunes the algorithms for more reasonable
performance; without much loss of quality.