The Translate node leaves empty pixels at the boundary of the image.
This caused by incorrect clipping when sampling the pixels. To fix this,
we adjust COM_MemoryBuffer::read to read using Extend or Repeat using
BLI interpolation, then multiply that by a clipping rectangle. The
read_elem_sampled function is now defined in terms of the read method.
This also coincidentally fixes off by half a pixel error in nearest
neighbour interpolation.
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