Offloading points and voxelmaps on GPU

[koide3]

This PR enables offloading point and voxelmap data of old submaps from GPU memory. It effectively reduces GPU memory consumption with a slight degradation of global optimization speed up to CPU-GPU data IO. A new parameter "gpu_memory_offload_mb" in "global_mapping_gpu.json" controls the threshold for GPU memory usage.

  // gpu_memory_offload_mb          : Threshold for GPU memory offloading in MB (0 = disable offloading).
  //                                : If GPU memory usage exceeds this value, old submap points and voxelmaps are offloaded to CPU memory.
  //                                : Note that the current implementation doesn't support offloading of factors and graphics data, and thus
  //                                : the actual GPU memory usage will be much larger than this value.
  //                                : (Setting this to a half or quarter of the total GPU memory would be a good choice).

TODO : Offloading in offline_viewer