Store information about a position to sample from an image. This class can be used to support a two-pass sampling approach where the first pass populates a list of SamplePositions and the second pass uses those positions to perform the actual sampling. The advantage of this approach is that an accurate bounding box can be calculated for the region we wish to sample from during the first pass. This allows us to create a tile of exactly the size we need when the sampler is created rather than having to default to the entire input request box.