Decoding: do not allocate memory for the codestream of each codeblock

Decoding: do not allocate memory for the codestream of each codeblock

Currently we allocate at least 8192 bytes for each codeblock, and copy
the relevant parts of the codestream in that per-codeblock buffer as we
decode packets.
As the whole codestream for the tile is ingested in memory and alive
during the decoding, we can directly point to it instead of copying. But
to do that, we need an intermediate concept, a 'chunk' of code-stream segment,
given that segments may be made of data at different places in the code-stream
when quality layers are used.

With that change, the decoding of MAPA_005.jp2 goes down from the previous
improvement of 2.7 GB down to 1.9 GB.

New profile:

n4: 1885648469 (heap allocation functions) malloc/new/new[], --alloc-fns, etc.
 n1: 1610689344 0x4E78287: opj_aligned_malloc (opj_malloc.c:61)
  n1: 1610689344 0x4E71D7B: opj_alloc_tile_component_data (tcd.c:676)
   n1: 1610689344 0x4E7272C: opj_tcd_init_decode_tile (tcd.c:816)
    n1: 1610689344 0x4E4BDD9: opj_j2k_read_tile_header (j2k.c:8618)
     n1: 1610689344 0x4E4C8A2: opj_j2k_decode_tiles (j2k.c:10349)
      n1: 1610689344 0x4E4E36E: opj_j2k_decode (j2k.c:7847)
       n1: 1610689344 0x4E52FA2: opj_jp2_decode (jp2.c:1564)
        n0: 1610689344 0x40374E: main (opj_decompress.c:1459)
 n1: 219232541 0x4E4BBF0: opj_j2k_read_tile_header (j2k.c:4685)
  n1: 219232541 0x4E4C8A2: opj_j2k_decode_tiles (j2k.c:10349)
   n1: 219232541 0x4E4E36E: opj_j2k_decode (j2k.c:7847)
    n1: 219232541 0x4E52FA2: opj_jp2_decode (jp2.c:1564)
     n0: 219232541 0x40374E: main (opj_decompress.c:1459)
 n1: 39822000 0x4E727A9: opj_tcd_init_decode_tile (tcd.c:1219)
  n1: 39822000 0x4E4BDD9: opj_j2k_read_tile_header (j2k.c:8618)
   n1: 39822000 0x4E4C8A2: opj_j2k_decode_tiles (j2k.c:10349)
    n1: 39822000 0x4E4E36E: opj_j2k_decode (j2k.c:7847)
     n1: 39822000 0x4E52FA2: opj_jp2_decode (jp2.c:1564)
      n0: 39822000 0x40374E: main (opj_decompress.c:1459)
 n0: 15904584 in 52 places, all below massif's threshold (1.00%)