hls: initial slice and partitioning framework
This commit provides a basic frame work for partitioning a frame into multiple slices, with the continued assumption of single-frame sequences. The decoder is modified to independently decode each slice and accumulate decoded points in a buffer for output. The encoder is updated to support partitioning the input point cloud into slices and to independently code each slice. Points from reconstructed slices are accumulated and output at the end of the frame period. The partitioning process (partitioning methods are defined in partitioning.cpp) proceeds as follows: - quantise the input point cloud without duplicate point removal or reordering points. - apply the partitioning function to produce a list of tiles and slices, each slice having an origin, id, and list of point indexes that identify points in the input point cloud. - producing a source point cloud for each partition as a subset of the input point cloud. - compressing each partition (slice) as normal by quantising the partitioned input. Recolouring is necessarily performed against the partitioned input since the recolouring method cannot correctly handle recolouring a partition from a complete point cloud. NB: this commit does not provide any partitioning methods.