Add a GeglSampler::interpolate() function, which subclasses should
implement if they use the generic box-filter algorithm.  This
function is similar to GeglSampler::get(), except that it always
performs point sampling (and therefore doesn't take a scale
matrix), and, in particular, should return its result using the
sampler's premultiplied interpolation format, rather than its
output format.

Use interpolate(), instead of get(), in _gegl_sampler_box_get(),
to avoid erroneously performing box filtering using the sampler's
output format.

Implement interpolate() in the linear and cubic samplers.